The failure to unregister the shrinker results will result in corruption
when the nfsd_net is freed.

Also clean up the drc_slab while we're here.

Reported-by:  <syzbot+83a43746cebef3508b49@syzkaller.appspotmail.com>
Fixes: db17b61765c2 ("nfsd4: drc containerization")
Signed-off-by: J. Bruce Fields <bfields@redhat.com>