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>