netfilter: xt_TCPMSS: fix handling of malformed TCP header and options

netfilter: xt_TCPMSS: fix handling of malformed TCP header and options

commit 71ffe9c77dd7a2b62207953091efa8dafec958dd upstream.

Make sure the packet has enough room for the TCP header and
that it is not malformed.

While at it, store tcph->doff*4 in a variable, as it is used
several times.

This patch also fixes a possible off by one in case of malformed
TCP options.

Reported-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>