kernfs: remove an unused if statement in kernfs_path_from_node_locked()

It makes no sense to call kernfs_path_from_node_locked() with NULL buf,
and no one is doing that right now.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20221126111634.1994-1-thunder.leizhen@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/kernfs/dir.c

@@ -149,9 +149,6 @@ static int kernfs_path_from_node_locked(struct kernfs_node *kn_to,
 	if (kn_from == kn_to)
 		return strlcpy(buf, "/", buflen);
 
-	if (!buf)
-		return -EINVAL;
-
 	common = kernfs_common_ancestor(kn_from, kn_to);
 	if (WARN_ON(!common))
 		return -EINVAL;