btrfs: remove pointless barrier from btrfs_sync_file()

The memory barrier (smp_mb()) at btrfs_sync_file() is completely redundant
now that fs_info->last_trans_committed is read using READ_ONCE(), with the
helper btrfs_get_last_trans_committed(), and written using WRITE_ONCE()
with the helper btrfs_set_last_trans_committed().

This barrier was introduced in 2011, by commit a4abeea4 ("Btrfs: kill
trans_mutex"), but even back then it was not correct since the writer side
(in btrfs_commit_transaction()), did not issue a pairing memory barrier
after it updated fs_info->last_trans_committed.

So remove this barrier.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>