UPSTREAM: netfilter: x_tables: validate e->target_offset early

(cherry pick from commit bdf533de6968e9686df777dc178486f600c6e617)

We should check that e->target_offset is sane before
mark_source_chains gets called since it will fetch the target entry
for loop detection.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Change-Id: Ic2dbc31c9525d698e94d4d8875886acf3524abbd
Bug: 29637687