netfilter: exthdr: add support for tcp option removal

This allows to replace a tcp option with nop padding to selectively disable
a particular tcp option.

Optstrip mode is chosen when userspace passes the exthdr expression with
neither a source nor a destination register attribute.

This is identical to xtables TCPOPTSTRIP extension.
The only difference is that TCPOPTSTRIP allows to pass in a bitmap
of options to remove rather than a single number.

Unlike TCPOPTSTRIP this expression can be used multiple times
in the same rule to get the same effect.

We could add a new nested attribute later on in case there is a
use case for single-expression-multi-remove.

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