udp: add a missing smp_wmb() in udp_lib_get_port()

Corey Minyard spotted a missing memory barrier in udp_lib_get_port()

We need to make sure a reader cannot read the new 'sk->sk_next' value
and previous value of 'sk->sk_hash'. Or else, an item could be deleted
from a chain, and inserted into another chain. If new chain was empty
before the move, 'next' pointer is NULL, and lockless reader can
not detect it missed following items in original chain.

This patch is temporary, since we expect an upcoming patch
to introduce another way of handling the problem.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>