udp: Set NULL to sk->sk_prot->h.udp_table.

We will soon introduce an optional per-netns hash table
for UDP.

This means we cannot use the global sk->sk_prot->h.udp_table
to fetch a UDP hash table.

Instead, set NULL to sk->sk_prot->h.udp_table for UDP and get
a proper table from net->ipv4.udp_table.

Note that we still need sk->sk_prot->h.udp_table for UDP LITE.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>