l2tp: fix UDP checksum support

The pppol2tp driver has had broken UDP checksum code for a long
time. This patch fixes it. If UDP checksums are enabled in the
tunnel's UDP socket, the L2TP driver now properly validates the
checksum on receive and fills in the checksum on transmit. If the
network device has hardware checksum support and is enabled, it is
used instead of generating/checking the checksum in software.

Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>