Don't try to reassemble IP_DF marked packets

Most modern TCP stacks mark TCP/IPv4 packets with IP_DF.
So, in addition to checking if the frags field is clear,
also check if the packet is marked as IP_DF, and bypass
reassembly if that is the case.

This improves perf by about ~150Mb/s for an rx heavy workload on a
10GbE NIC.

Signed-off-by: Andrew Gallatin <gallatin@google.com>