nfsd: move reply cache initialization into nfsd startup

There's no need to start the reply cache before nfsd is up and running,
and doing so means that we register a shrinker for every net namespace
instead of just the ones where nfsd is running.

Move it to the per-net nfsd startup instead.

Reported-by: Dai Ngo <dai.ngo@oracle.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>