net: Convert nfs4_dns_resolver_ops

These pernet_operations look similar to rpcsec_gss_net_ops,
they just create and destroy another cache. Also they create
and destroy directory. So, they also look safe to be async.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Acked-by: Anna Schumaker <Anna.Schumaker@netapp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>