fs: nfsd: Fix signedness bug in compare_blob

Bugs similar to the one in acbbe6fb (kcmp: fix standard comparison
bug) are in rich supply.

In this variant, the problem is that struct xdr_netobj::len has type
unsigned int, so the expression o1->len - o2->len _also_ has type
unsigned int; it has completely well-defined semantics, and the result
is some non-negative integer, which is always representable in a long
long. But this means that if the conditional triggers, we are
guaranteed to return a positive value from compare_blob.

In this case it could be fixed by

-       res = o1->len - o2->len;
+       res = (long long)o1->len - (long long)o2->len;

but I'd rather eliminate the usually broken 'return a - b;' idiom.
Reviewed-by: Jeff Layton <jlayton@primarydata.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4state.c

@@ -1714,15 +1714,14 @@ static int copy_cred(struct svc_cred *target, struct svc_cred *source)
 	return 0;
 }
 
-static long long
+static int
 compare_blob(const struct xdr_netobj *o1, const struct xdr_netobj *o2)
 {
-	long long res;
-
-	res = o1->len - o2->len;
-	if (res)
-		return res;
-	return (long long)memcmp(o1->data, o2->data, o1->len);
+	if (o1->len < o2->len)
+		return -1;
+	if (o1->len > o2->len)
+		return 1;
+	return memcmp(o1->data, o2->data, o1->len);
 }
 
 static int same_name(const char *n1, const char *n2)
@@ -1910,7 +1909,7 @@ add_clp_to_name_tree(struct nfs4_client *new_clp, struct rb_root *root)
 static struct nfs4_client *
 find_clp_in_name_tree(struct xdr_netobj *name, struct rb_root *root)
 {
-	long long cmp;
+	int cmp;
 	struct rb_node *node = root->rb_node;
 	struct nfs4_client *clp;