netfilter: rt: add support to fetch path mss

to be used in combination with tcp option set support to mimic
iptables TCPMSS --clamp-mss-to-pmtu.

v2: Eric Dumazet points out dst must be initialized.

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