iflib: Take iri_pad into account when processing small frames

[kd]

Summary

Drivers can specify padding of received frames with iri_pad field.
This can be used to enforce ip alignment by hardware.
Iflib ignores that padding when processing small frames, which renders this feature inoperable.
I found it while writing a driver for a NIC that can ip align received packets.
Note that this doesn't change behavior of existing drivers as they all set iri_pad to 0.