Revert "udp: remove redundant variable"

This reverts commit 81d54ec8479a2c695760da81f05b5a9fb2dbe40a.

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>