nfsd: don't destroy global nfs4_file table in per-net shutdown

The nfs4_file table is global, so shutting it down when a containerized
nfsd is shut down is wrong and can lead to double-frees. Tear down the
nfs4_file_rhltable in nfs4_state_shutdown instead of
nfs4_state_shutdown_net.

Fixes: d47b295e

 ("NFSD: Use rhashtable for managing nfs4_file objects")
Link: https://bugzilla.redhat.com/show_bug.cgi?id=2169017
Reported-by: JianHong Yin <jiyin@redhat.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>