sunrpc: Use uid_eq and gid_eq where appropriate

When comparing uids use uid_eq instead of ==.
When comparing gids use gid_eq instead of ==.

And unfortunate cost of type safety.

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