netfilter: x_tables: validate e->target_offset early

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>
Signed-off-by: Dennis Cagle <d-cagle@codeaurora.org>
Git-repo: http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git
Git-commit: bdf533de6968e9686df777dc178486f600c6e617
(cherry picked from commit bdf533de6968e9686df777dc178486f600c6e617)
Change-Id: Id3ec56cdc333990d62c99d6c2e59dbcce633bcc1