zygo: cherry-picked up to e56a951 (extra args linus/master --remotes=btrfs-next/...

zygo: cherry-picked up to e56a951 (extra args linus/master --remotes=btrfs-next/* --remotes=mason/* --after=now - 90 days -- fs/btrfs)

Commits included:

---- 83fe27e rcu: Make SRCU optional by using CONFIG_SRCU
---- b4caecd fs: introduce f_op->mmap_capabilities for nommu mmap support
---- de1414a fs: export inode_to_bdi and use it in favor of mapping->backing_dev_info
---- b83ae6d fs: remove mapping->backing_dev_info
---- df0ce26 fs: remove default_backing_dev_info
---- d83a08d mm: drop vm_ops->remap_pages and generic_file_remap_pages() stub
---- 8d38633 page_writeback: put account_page_redirty() after set_page_dirty()
---- b7a0365 Btrfs: ctree: reduce args where only fs_info used
---- a585e94 Btrfs: delayed-inode: replace root args iff only fs_info used
---- 01d5847 Btrfs: disk-io: replace root args iff only fs_info used
---- f749303 Btrfs: switch to kvfree() helper
---- e8c9f18 btrfs: constify structs with op functions or static definitions
---- b9fd47c btrfs: cleanup: remove no-used alloc_chunk in btrfs_check_data_free_space()
---- a688a04 btrfs: remove unused chunk_tree argument in several functions
---- 08da757 btrfs: cleanup: use for() loop in btrfs_map_bio()
---- 41d6b13 btrfs: fix sizeof format specifier in btrfs_check_super_valid()
---- 9eaed21 btrfs: remove unused fs_info arg from btrfs_close_extra_devices()
---- 0489234 btrfs: consistently use fs_info in close_ctree()
---- 638aa7e btrfs: factor btrfs_init_scrub() out of open_ctree()
---- 779a65a btrfs: factor btrfs_init_balance() out of open_ctree()
---- f37938e btrfs: factor btrfs_init_btree_inode() out of open_ctree()
---- ad61836 btrfs: factor btrfs_init_dev_replace_locks() out of open_ctree()
---- f9e92e4 btrfs: factor btrfs_init_qgroup() out of open_ctree()
---- 2a45819 btrfs: factor btrfs_init_workqueues() out of open_ctree()
---- 63443bf btrfs: factor btrfs_replay_log() out of open_ctree()
---- 4bbcaa6 btrfs: factor btrfs_read_roots() out of open_ctree()
---- 6f01105 btrfs: use correct type for workqueue flags
---- a4f3d2c btrfs: cleanup, reduce temporary variables in btrfs_read_roots
---- e36cb0b VFS: (Scripted) Convert S_ISLNK/DIR/REG(dentry->d_inode) to d_is_*(dentry)
---- 853d8ec btrfs: need_resched not needed with cond_resched
---- 351810c btrfs: use cond_resched_lock where possible
---- f8c269d btrfs: cleanup 64bit/32bit divs, compile time constants
---- 31e818f btrfs: cleanup, use kmalloc_array/kcalloc array helpers
---- f64c7b1 btrfs: remove shadowing variables in __btrfs_buffered_write
---- 3284da7 btrfs: use explicit initializer for seq_elem
---- b8b93ad btrfs: cleanup 64bit/32bit divs, provably bounded values
---- 47c5713 btrfs: replace remaining do_div calls with div_u64 variants
---- 9d644a6 btrfs: cleanup, use correct type in div_u64_rem
---- 093adbc btrfs: switch helper macros to static inlines in sysfs.h
---- 258ece0 btrfs: remove shadowing variables in __btrfs_map_block
---- 9266ff8 Btrfs: fix ASSERT(list_empty(&cur_trans->dirty_bgs_list)
---- aa4661f Btrfs: prepare block group cache before writing
---- 2e0adf0 fix qgroups
---- 777414a Btrfs: fix merge delalloc logic
---- 3075d58 Btrfs: account for the correct number of extents for delalloc reservations
---- 2fdbf36 Btrfs: just free dummy extent buffers
---- 464919a Btrfs: add sanity test for outstanding_extents accounting
++++ 4f764e5 Btrfs: remove deleted xattrs on fsync log replay
++++ 5cc2b17 Btrfs: send, add missing check for dead clone root
++++ 2f1f465 Btrfs: send, don't leave without decrementing clone root's send_progress
++++ 8996a48 Btrfs: incremental send, clear name from cache after orphanization
++++ 5f806c3 Btrfs: incremental send, remove dead code
++++ 3d850dd Btrfs: add missing inode item update in fallocate()
++++ bf69196 Btrfs: change the insertion criteria for the qgroup operations rbtree
++++ 2f2ff0e Btrfs: fix metadata inconsistencies after directory fsync
++++ dcc82f4 Btrfs: fix log tree corruption when fs mounted with -o discard
++++ 3c3b04d btrfs: don't accept bare namespace as a valid xattr
++++ 26e726a btrfs: incorrect handling for fiemap_fill_next_extent return
++++ d862095 btrfs: explicitly set control file's private_data
++++ 727b978 btrfs: cleanup orphans while looking up default subvolume
++++ e56a951 Btrfs: Remove the check for old-style mkfs