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

(cherry picked 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.

Bug: 29637687
Change-Id: Ida3d7055b4905cf4c18550f3989f529a8ff5e8a9
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Mekala Natarajan <mnatarajan@google.com>