[IPV4] udp: trim forgets about CHECKSUM_HW

A UDP packet may contain extra data that needs to be trimmed off.
But when doing so, UDP forgets to fixup the skb checksum if CHECKSUM_HW
is being used.

I think this explains the case of a NFS receive using skge driver
causing 'udp hw checksum failures' when interacting with a crufty
settop box.

Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>