Revert "udp: remove redundant variable"

Revert "udp: remove redundant variable"

This reverts commit d8062aa7f9e7962e2a0c09868e887770cc26f2e0.

If we take the "try_again" goto, due to a checksum error,
the 'len' has already been truncated.  So we won't compute
the same values as the original code did.

Reported-by: paul bilke <fsmail@conspiracy.net>
Signed-off-by: David S. Miller <davem@davemloft.net>