btrfs: do commit in sync_fs if there are pending changes

If a pending change is requested, it's not processed unless there is a
transaction commit about to happen, not even after sync or SYNC_FS
ioctl. For example a remount that toggles the inode_cache option will
not take effect after sync on a quiescent filesystem.

Signed-off-by: David Sterba <dsterba@suse.cz>