knfsd: fix callback rpc cred

It doesn't make sense to make the callback with credentials that the
client made the setclientid with.  Instead the spec requires that the
callback occur with the credentials the client authenticated *to*.
It probably doesn't matter what we use for auth_unix, and some more
infrastructure will be needed for auth_gss, so let's just remove the
cred lookup for now.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Acked-by: Neil Brown <neilb@suse.de>