bcachefs: .get_parent() should return an error pointer

Delete the useless check for inum == 0; we'll return -ENOENT without it,
which is what we want.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/fs.c

@@ -1213,9 +1213,6 @@ static struct dentry *bch2_get_parent(struct dentry *child)
 		.inum = inode->ei_inode.bi_dir,
 	};
 
-	if (!parent_inum.inum)
-		return NULL;
-
 	return d_obtain_alias(bch2_vfs_inode_get(c, parent_inum));
 }