nfsd: dynamically allocate DRC entries

The existing code keeps a fixed-size cache of 1024 entries. This is much
too small for a busy server, and wastes memory on an idle one.  This
patch changes the code to dynamically allocate and free these cache
entries.

A cap on the number of entries is retained, but it's much larger than
the existing value and now scales with the amount of low memory in the
machine.

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