netfilter: Add of NATTYPE COOKIE Check

Add NATTYPE COOKIE to avoid invalid access of NATTYPE
entry from the conntrack module. This compensates for
possible garbage values in the conntrack entry, which
would cause potential errors.

Change-Id: I1a53fa0dc6961dd3e53d382642b413d4ee781ed6
Signed-off-by: Tyler Wear <twear@codeaurora.org>
Signed-off-by: Devendra Patel <cdevenp@codeaurora.org>