SUNRPC: Fix machine creds in generic_create_cred and generic_match

- generic_create_cred needs to copy the '.principal' field.
- generic_match needs to ignore the groups and match on the '.principal'
  field.

This fixes an Oops that was introduced by commit 68c97153 (SUNRPC:
Clean up the RPCSEC_GSS service ticket requests)

Reported-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Tested-by: J. Bruce Fields <bfields@redhat.com>