netfilter: xt_TCPMSS: add more sanity tests on tcph->doff

netfilter: xt_TCPMSS: add more sanity tests on tcph->doff

Orabug: 27896802
CVE: CVE-2017-18017

Denys provided an awesome KASAN report pointing to an use
after free in xt_TCPMSS

I have provided three patches to fix this issue, either in xt_TCPMSS or
in xt_tcpudp.c. It seems xt_TCPMSS patch has the smallest possible
impact.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Denys Fedoryshchenko <nuclearcat@nuclearcat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
(cherry picked from commit 2638fd0f92d4397884fd991d8f4925cb3f081901)
Signed-off-by: Brian Maly <brian.maly@oracle.com>