nfsd: update percpu_ref to manage references on nfsd_net

Holding a reference on nfsd_net is what is required, it was never
actually about ensuring nn->nfsd_serv available.

Move waiting for outstanding percpu references from
nfsd_destroy_serv() to nfsd_shutdown_net().

By moving it later it will be possible to invalidate localio clients
during nfsd_file_cache_shutdown_net() via __nfsd_file_cache_purge().

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Acked-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>