nlm_shutdown_hosts_net() cleanup

nlm_complain_hosts() walks through nlm_server_hosts hlist, which should
be protected by nlm_host_mutex.

Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>