exthdr: prefer raw_type instead of desc->type

exthdr: prefer raw_type instead of desc->type

On ancient kernels desc can be NULL, because such kernels do not
understand NFTA_EXTHDR_TYPE.

Thus they don't include it in the reverse dump, so the tcp/ip
option gets treated like an ipv6 exthdr, but no matching
description will be found.

This then gives a crash due to the null deref.

Just use the raw value here, this avoid a crash and at least
print *something*, e.g.:

unknown-exthdr unknown & 0xf0 [invalid type] == 0x0 [invalid type]

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