GSO/UDP: Use skb->len instead of udph->len to determine length of original skb

It is possible for tunnels to end up generating IP or IPv6 datagrams that
are larger than 64K and expecting to be segmented.  As such we need to deal
with length values greater than 64K.  In order to accommodate this we need
to update the code to work with a 32b length value instead of a 16b one.

Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>