[UDP]: Revert 2-pass hashing changes.

This reverts changesets:

6aaf47fa
b7b5f487
de34ed91
fc038410



There are still some correctness issues recently
discovered which do not have a known fix that doesn't
involve doing a full hash table scan on port bind.

So revert for now.

Signed-off-by: David S. Miller <davem@davemloft.net>