vtnet: Better adjust for ethernet alignment.

Move adjustment of the mbuf from where we allocate it to where we are
about to queue it to the device. Do this only on those platforms that
require it. This allows us to receive an entire jumbo frame on other
platforms. It also doesn't make the adjustment on subsequent frames when
we queue mulitple mbufs for LRO operations.

For the normal use case on armv7, there's no difference because we only
ever allocate one mbuf. However, for the LRO cases it increases what's
available in LRO. It also ensure that we get enough mbufs in those cases
as well (though I have no ability to test this on a LRO scenario with
armv7).

This has the side effect of reverting 527b62e37e68.

Fixes: 527b62e37e68
Sponsored by: Netflix