igb: packet-split descriptors support

Packet-split descriptors are used by Linux VF driver for MTU values from 2048

Signed-off-by: Tomasz Dzieciol <t.dzieciol@partner.samsung.com>
Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Tested-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>