netfilter: change generic l4 protocol number

0 is used by Hop-by-hop header and so this may cause confusion.
255 is stated as 'Reserved' by IANA.

Signed-off-by: Christoph Paasch <christoph.paasch@student.uclouvain.be>
Signed-off-by: Patrick McHardy <kaber@trash.net>