netlink: Reset portid after netlink_insert failure

netlink: Reset portid after netlink_insert failure

The commit ef1875751f785b94f3599d6bb558e5defc8d1d91 ("netlink:
eliminate nl_sk_hash_lock") breaks the autobind retry mechanism
because it doesn't reset portid after a failed netlink_insert.

This means that should autobind fail the first time around, then
the socket will be stuck in limbo as it can never be bound again
since it already has a non-zero portid.

Fixes: ef1875751f78 ("netlink: eliminate nl_sk_hash_lock")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>