netfilter: conntrack: clamp l4proto array size at largers supported protocol

All higher l4proto numbers are handled by the generic tracker; the
l4proto lookup function already returns generic one in case the l4proto
number exceeds max size.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>