net: sunrpc: interpret the return value of kstrtou32 correctly

A return value of 0 means success. This is documented in lib/kstrtox.c.

This was found by trying to mount an NFS share from a link-local IPv6
address with the interface specified by its index:

  mount("[fe80::1%1]:/srv/nfs", "/mnt", "nfs", 0, "nolock,addr=fe80::1%1")

Before this commit this failed with EINVAL and also caused the following
message in dmesg:

  [...] NFS: bad IP address specified: addr=fe80::1%1

The syscall using the same address based on the interface name instead
of its index succeeds.

Credits for this patch go to my colleague Christian Speich, who traced
the origin of this bug to this line of code.
Signed-off-by: NJohannes Nixdorf <j.nixdorf@avm.de>
Fixes: 00cfaa94 ("replace strict_strto calls")
Signed-off-by: NTrond Myklebust <trond.myklebust@hammerspace.com>

net/sunrpc/addr.c

@@ -185,7 +185,7 @@ static int rpc_parse_scope_id(struct net *net, const char *buf,
 			scope_id = dev->ifindex;
 			dev_put(dev);
 		} else {
-			if (kstrtou32(p, 10, &scope_id) == 0) {
+			if (kstrtou32(p, 10, &scope_id) != 0) {
 				kfree(p);
 				return 0;
 			}