sunrpc: Simplify auth_unix now that everything is a kgid_t

In unx_create_cred directly assign gids from acred->group_info
to cred->uc_gids.

In unx_match directly compare uc_gids with group_info.

Now that both group_info and unx_cred gids are stored as kgids
this is valid and the extra layer of translation can be removed.

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