nfsd: make unconf_id_hashtbl allocated per net

This hash holds nfs4_clients info, which are network namespace aware.
So let's make it allocated per network namespace.

Note: this hash can be allocated in per-net operations. But it looks
better to allocate it on nfsd state start and thus don't waste resources
if server is not running.

Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>