nfsd4: don't try to map gid's in generic rpc code

For nfsd we provide users the option of mapping uid's to server-side
supplementary group lists.  That makes sense for nfsd, but not
necessarily for other rpc users (such as the callback client).

So move that lookup to svcauth_unix_set_client, which is a
program-specific method.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>