netfilter: conntrack: use NFPROTO_MAX to size array

We don't support anything larger than NFPROTO_MAX, so we can shrink this a bit:

     text data  dec  hex filename
old: 8259 1096 9355 248b net/netfilter/nf_conntrack_proto.o
new: 8259  624 8883 22b3 net/netfilter/nf_conntrack_proto.o

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