nfsd: disable deferral for NFSv4

Now that a slight delay in getting a reply to an upcall doesn't
require deferring of requests, request deferral for all NFSv4
requests - the concept doesn't really fit with the v4 model.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>