l2tp: fix data packet sequence number handling

If enabled, L2TP data packets have sequence numbers which a receiver
can use to drop out of sequence frames or try to reorder them. The
first frame has sequence number 0, but the L2TP code currently expects
it to be 1. This results in the first data frame being handled as out
of sequence.

This one-line patch fixes the problem.

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