selftests: netfilter: fix udpclash tool hang

Yi Chen reports that 'udpclash' loops forever depending on compiler
(and optimization level used); while (x == 1) gets optimized into
for (;;).  Add volatile qualifier to avoid that.

While at it, also run it under timeout(1) and fix the resize script
to not ignore the timeout passed as second parameter to insert_flood.

Reported-by: Yi Chen <yiche@redhat.com>
Suggested-by: Yi Chen <yiche@redhat.com>
Fixes: 78a58836 ("selftests: netfilter: add conntrack clash resolution test case")
Signed-off-by: Florian Westphal <fw@strlen.de>