nfsd4: expire clients more promptly

Expire clients more promptly, at the expense of possibly running the
laundromat thread more frequently.

Though it's not the default, I'd like it to be feasible to run with a
lease time of just a few seconds, at which point a minimum 10 second
wait between laundromat runs seems a little much.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>