Btrfs: remove BUG_ON in start_transaction

Since we could get errors from the concurrent aborted transaction,
the check of this BUG_ON in start_transaction is not true any more.

Say, while flushing free space cache inode's dirty pages,
btrfs_finish_ordered_io
 -> btrfs_join_transaction_nolock
      (the transaction has been aborted.)
      -> BUG_ON(type == TRANS_JOIN_NOLOCK);

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/transaction.c

@@ -549,11 +549,8 @@ again:
 		}
 	} while (ret == -EBUSY);
 
-	if (ret < 0) {
-		/* We must get the transaction if we are JOIN_NOLOCK. */
-		BUG_ON(type == TRANS_JOIN_NOLOCK);
+	if (ret < 0)
 		goto join_fail;
-	}
 
 	cur_trans = root->fs_info->running_transaction;