netfilter: x_tables: cap allocations at 512 mbyte

Arbitrary limit, however, this still allows huge rulesets
(> 1 million rules).  This helps with automated testing as it prevents
oom-killer invocation.

Change-Id: Ide29e7bfccc68996a4dc45ac834b4645902601c1
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Git-commit: 19926968ea86a286aa6fbea16ee3f2e7442f10f0
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/davem/net.git


Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>
Signed-off-by: Chinmay Agarwal <chinagar@codeaurora.org>