udp: Access &udp_table via net.

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

This means we cannot use udp_table directly in most places.

Instead, access it via net->ipv4.udp_table.

The access will be valid only while initialising udp_table
itself and creating/destroying each netns.

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