btrfs: introduce pending action: commit

btrfs: introduce pending action: commit

In some contexts, like in sysfs handlers, we don't want to trigger a
transaction commit. It's a heavy operation, we don't know what external
locks may be taken. Instead, make it possible to finish the operation
through sync syscall or SYNC_FS ioctl.

Signed-off-by: David Sterba <dsterba@suse.cz>
(cherry picked from commit d51033d05547675f898ce4233a7d8d1a0dfe2984)