sunrpc: Use gid_valid to test for gid != INVALID_GID

In auth unix there are a couple of places INVALID_GID is used a
sentinel to mark the end of uc_gids array.  Use gid_valid
as a type safe way to verify we have not hit the end of
valid data in the array.

Cc: "J. Bruce Fields" <bfields@fieldses.org>
Cc: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

net/sunrpc/auth_unix.c

@@ -131,8 +131,7 @@ unx_match(struct auth_cred *acred, struct rpc_cred *rcred, int flags)
 	for (i = 0; i < groups ; i++)
 		if (!gid_eq(cred->uc_gids[i], GROUP_AT(acred->group_info, i)))
 			return 0;
-	if (groups < NFS_NGROUPS &&
-	    cred->uc_gids[groups] != INVALID_GID)
+	if (groups < NFS_NGROUPS && gid_valid(cred->uc_gids[groups]))
 		return 0;
 	return 1;
 }
@@ -161,7 +160,7 @@ unx_marshal(struct rpc_task *task, __be32 *p)
 	*p++ = htonl((u32) cred->uc_uid);
 	*p++ = htonl((u32) cred->uc_gid);
 	hold = p++;
-	for (i = 0; i < 16 && cred->uc_gids[i] != INVALID_GID; i++)
+	for (i = 0; i < 16 && gid_valid(cred->uc_gids[i]); i++)
 		*p++ = htonl((u32) cred->uc_gids[i]);
 	*hold = htonl(p - hold - 1);		/* gid array length */
 	*base = htonl((p - base - 1) << 2);	/* cred length */