btrfs: simplify code flow in btrfs_delayed_inode_reserve_metadata

btrfs_block_rsv_add can return only ENOSPC since it's called with
NO_FLUSH modifier. This so simplify the logic in
btrfs_delayed_inode_reserve_metadata to exploit this invariant.
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ add assert and comment ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/delayed-inode.c

@@ -632,29 +632,14 @@ static int btrfs_delayed_inode_reserve_metadata(
 			return ret;
 		ret = btrfs_block_rsv_add(root, dst_rsv, num_bytes,
 					  BTRFS_RESERVE_NO_FLUSH);
-		/*
-		 * Since we're under a transaction reserve_metadata_bytes could
-		 * try to commit the transaction which will make it return
-		 * EAGAIN to make us stop the transaction we have, so return
-		 * ENOSPC instead so that btrfs_dirty_inode knows what to do.
-		 */
-		if (ret == -EAGAIN) {
-			ret = -ENOSPC;
+		/* NO_FLUSH could only fail with -ENOSPC */
+		ASSERT(ret == 0 || ret == -ENOSPC);
+		if (ret)
 			btrfs_qgroup_free_meta_prealloc(root, num_bytes);
-		}
-		if (!ret) {
-			node->bytes_reserved = num_bytes;
-			trace_btrfs_space_reservation(fs_info,
-						      "delayed_inode",
-						      node->inode_id,
-						      num_bytes, 1);
 	} else {
-			btrfs_qgroup_free_meta_prealloc(root, num_bytes);
-		}
-		return ret;
+		ret = btrfs_block_rsv_migrate(src_rsv, dst_rsv, num_bytes, true);
 	}
 
-	ret = btrfs_block_rsv_migrate(src_rsv, dst_rsv, num_bytes, true);
 	if (!ret) {
 		trace_btrfs_space_reservation(fs_info, "delayed_inode",
 					      node->inode_id, num_bytes, 1);