nfsd: don't try to reuse an expired DRC entry off the list

Currently when we are processing a request, we try to scrape an expired
or over-limit entry off the list in preference to allocating a new one
from the slab.

This is unnecessarily complicated. Just use the slab layer.

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