netfilter: add IPv4/6 IPComp extension match support

With this plugin, user could specify IPComp tagged with certain
CPI that host not interested will be DROPped or any other action.

For example:
iptables  -A INPUT -p 108 -m ipcomp --ipcompspi 0x87 -j DROP
ip6tables -A INPUT -p 108 -m ipcomp --ipcompspi 0x87 -j DROP

Then input IPComp packet with CPI equates 0x87 will not reach
upper layer anymore.

Signed-off-by: Fan Du <fan.du@windriver.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>