nfsd: update workqueue creation

No real change in functionality, but the old interface seems to be
deprecated.

We don't actually care about ordering necessarily, but we do depend on
running at most one work item at a time: nfsd4_process_cb_update()
assumes that no other thread is running it, and that no new callbacks
are starting while it's running.

Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>