macvlan/macvtap: Add support for L2 forwarding offloads with macvtap

This patch reverts earlier commit b13ba1b8 ("macvlan: forbid L2
fowarding offload for macvtap"). The reason for reverting this is because
the original patch no longer fixes what it previously did as the
underlying structure has changed for macvtap. Specifically macvtap
originally pulled packets directly off of the lowerdev. However in commit
6acf54f1 ("macvtap: Add support of packet capture on macvtap device.")
that code was changed and instead macvtap would listen directly on the
macvtap device itself instead of the lower device. As such, the L2
forwarding offload should now be able to provide a performance advantage of
skipping the checks on the lower dev while not introducing any sort of
regression.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>