zygo: cherry-picked up to e36cb0b (extra args linus/master --remotes=btrfs-next/...

Commits included:

---- d43955b Btrfs: change how we track dirty roots
---- fe7944d Btrfs: track dirty block groups on their own list
---- 7031479 Btrfs: abort transaction if we don't find the block group
---- 83fe27e rcu: Make SRCU optional by using CONFIG_SRCU
---- 98bd5c5 btrfs: sync ioctl, handle errors after transaction start
---- 379d685 Btrfs: fix incorrect freeing in scrub_stripe
---- 75c68e9 Btrfs: fix race deleting block group from space_info->ro_bgs list
---- 6e1103a btrfs: fix state->private cast on 32 bit machines
---- 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
---- 6c9fe14 btrfs: Fix the bug that fs_info->pending_changes is never cleared.
---- a53f4f8 btrfs: Don't call btrfs_start_transaction() on frozen fs to avoid deadlock.
++++ 063c54d btrfs: fix raid56 scrub failed in xfstests btrfs/072
---- 3da5ab5 Btrfs: add missing blk_finish_plug in btrfs_sync_log()
---- d83a08d mm: drop vm_ops->remap_pages and generic_file_remap_pages() stub
---- 8d38633 page_writeback: put account_page_redirty() after set_page_dirty()
---- e36cb0b VFS: (Scripted) Convert S_ISLNK/DIR/REG(dentry->d_inode) to d_is_*(dentry)