net: flow_dissector: fail on evil iph->ihl

We don't validate iph->ihl which may lead a dead loop if we meet a IPIP
skb whose iph->ihl is zero. Fix this by failing immediately when iph->ihl
is evil (less than 5).

CRs-Fixed: 589913
Change-Id: Ifd7be75d153d7504704e7ff9d8f63fe3767326d2
Acked-by: David Arinzon <darinzon@qti.qualcomm.com>
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>