Btrfs: remove unnecessary memory barrier in btrfs_sync_log()

Mutex unlock implies certain memory barriers to make sure all the memory
operation completes before the unlock, and the next mutex lock implies memory
barriers to make sure the all the memory happens after the lock. So it is
a full memory barrier(smp_mb), we needn't add memory barriers. Remove them.
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>

fs/btrfs/tree-log.c

@@ -2496,7 +2496,6 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
 	root->log_transid++;
 	log->log_transid = root->log_transid;
 	root->log_start_pid = 0;
-	smp_mb();
 	/*
 	 * IO has been started, blocks of the log tree have WRITTEN flag set
 	 * in their headers. new modifications of the log will be written to
@@ -2589,8 +2588,6 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
 				btrfs_header_level(log_root_tree->node));
 
 	log_root_tree->log_transid++;
-	smp_mb();
-
 	mutex_unlock(&log_root_tree->log_mutex);
 
 	/*