kernfs: Fix kernfs_name_compare

Returning a difference from a comparison functions is usually wrong
(see acbbe6fb "kcmp: fix standard comparison bug" for the long
story). Here there is the additional twist that if the void pointers
ns and kn->ns happen to differ by a multiple of 2^32,
kernfs_name_compare returns 0, falsely reporting a match to the
caller.

Technically 'hash - kn->hash' is ok since the hashes are restricted to
31 bits, but it's better to avoid that subtlety.
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/kernfs/dir.c

@@ -201,10 +201,14 @@ static unsigned int kernfs_name_hash(const char *name, const void *ns)
 static int kernfs_name_compare(unsigned int hash, const char *name,
 			       const void *ns, const struct kernfs_node *kn)
 {
-	if (hash != kn->hash)
-		return hash - kn->hash;
-	if (ns != kn->ns)
-		return ns - kn->ns;
+	if (hash < kn->hash)
+		return -1;
+	if (hash > kn->hash)
+		return 1;
+	if (ns < kn->ns)
+		return -1;
+	if (ns > kn->ns)
+		return 1;
 	return strcmp(name, kn->name);
 }