btrfs: Move error handling of btrfs_start_dirty_block_groups closer to call site

Even though btrfs_start_dirty_block_groups is fairly in the beginning of
btrfs_commit_transaction outside of the critical section defined by the
transaction states it can only be run by a single comitter. In other
words it defines its own critical section thanks to the
BTRFS_TRANS_DIRTY_BG run flag and ro_block_group_mutex. However, its
error handling is outside of this critical section which is a bit
counter-intuitive. So move the error handling righ after the function is
executed and let the sole runner of dirty block groups handle the return
value. No functional changes.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/transaction.c

@@ -2013,12 +2013,13 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans)
 			run_it = 1;
 		mutex_unlock(&fs_info->ro_block_group_mutex);
 
-		if (run_it)
+		if (run_it) {
 			ret = btrfs_start_dirty_block_groups(trans);
-	}
-	if (ret) {
-		btrfs_end_transaction(trans);
-		return ret;
+			if (ret) {
+				btrfs_end_transaction(trans);
+				return ret;
+			}
+		}
 	}
 
 	spin_lock(&fs_info->trans_lock);