netfilter: seqadj: re-load tcp header pointer after possible head reallocation

When adjusting sack block sequence numbers, skb_make_writable() gets
called to make sure tcp options are all in the linear area, and buffer
is not shared.

This can cause tcp header pointer to get reallocated, so we must
reaload it to avoid memory corruption.

This bug pre-dates git history.

Reported-by: Neel Mehta <nmehta@google.com>
Reported-by: Shane Huntley <shuntley@google.com>
Reported-by: Heather Adkins <argv@google.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>