src: handle rt0 and rt2 properly

src: handle rt0 and rt2 properly

Type 0 and 2 of the IPv6 Routing extension header are not handled
properly by exthdr_init_raw() in src/exthdr.c

In order to fix the bug, we extended the "enum nft_exthdr_op" to
differentiate between rt, rt0, and rt2.

This patch should fix the bug. We tested the patch against the
same configuration reported in the bug and the output is as
shown below.

table ip6 filter {
	chain input {
		type filter hook input priority 0; policy accept;
		rt0 addr[1] a::2
	}
}

Fixes: Bugzilla #1219
Signed-off-by: Ahmed Abdelsalam <amsalam20@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>