netfilter: nf_flow_table: ignore DF bit setting

Its irrelevant if the DF bit is set or not, we must pass packet to
stack in either case.

If the DF bit is set, we must pass it to stack so the appropriate
ICMP error can be generated.

If the DF is not set, we must pass it to stack for fragmentation.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>