other operating systems always set the "don't fragment" bit
in ther outgoing ipv4 packets causing us to unnecesarily
call ip4reassemble() looking for a fragment reassembly queue.
the change excludes the "don't fragment" bit from the test
so we now call ip4reassemble() only when the "more fragmens"
bit is set or a fragment offset other than zero is given.
this optimization was discovered from akaros.
1db9f19b62