netfilter: rt: account for tcp header size too

This needs to accout for the ipv4/ipv6 header size and the tcp
header without options.

Fixes: 6b5dc98e ("netfilter: rt: add support to fetch path mss")
Reported-by: Matteo Croce <technoboy85@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>