nfsd: set timeparms.to_maxval in setup_callback_client

...otherwise the logic in the timeout handling doesn't work correctly.

Spotted-by: Trond Myklebust <trond.myklebust@primarydata.com>
Cc: stable@vger.kernel.org
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>