src: avoid IPPROTO_MAX for array definitions

src: avoid IPPROTO_MAX for array definitions

ip header can only accomodate 8but value, but IPPROTO_MAX has been bumped
due to uapi reasons to support MPTCP (262, which is used to toggle on
multipath support in tcp).

This results in:
exthdr.c:349:11: warning: result of comparison of constant 263 with expression of type 'uint8_t' (aka 'unsigned char') is always true [-Wtautological-constant-out-of-range-compare]
if (type < array_size(exthdr_protocols))
            ~~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~

redude array sizes back to what can be used on-wire.

Signed-off-by: Florian Westphal <fw@strlen.de>