ibmvnic: Pad small packets to minimum MTU size

Some backing devices cannot handle small packets well,
so pad any small packets to avoid that. It was recommended
that the VNIC driver should not send packets smaller than the
minimum MTU value provided by firmware, so pad small packets
to be at least that long.

Signed-off-by: Thomas Falcon <tlfalcon@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>