netfilter: contrack: Adding check for SIP/2.0

Packets arriving at SIP port will now be checked for SIP/2.0 before
getting tagged as SIP packet. Added helper functions strnstr and strnstrn
for checking substring in a string in case of non null termination.

Change-Id: Ia7d6a3ac5dd1e5f0ce97ee9f6d0c13f16d9dd7f2
Acked-by: Vinisha Varre <vvarre@qti.qualcomm.com>
Signed-off-by: Paras Singh Jain <parassin@codeaurora.org>