nfsd: register a shrinker for DRC cache entries

Since we dynamically allocate them now, allow the system to call us up
to release them if it gets low on memory. Since these entries aren't
replaceable, only free ones that are expired or that are over the cap.
The the seeks value is set to '1' however to indicate that freeing the
these entries is low-cost.

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