auth_gss: fetch the acceptor name out of the downcall

If rpc.gssd sends us an acceptor name string trailing the context token,
stash it as part of the context.

Signed-off-by: Jeff Layton <jlayton@poochiereds.net>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>