veth: implement support for set_channel ethtool op

This change implements the set_channel() ethtool operation,
preserving the current defaults values and allowing up set
the number of queues in the range set ad device creation
time.

The update operation tries hard to leave the device in a
consistent status in case of errors.

RFC v1 -> RFC v2:
 - don't flip device status on set_channel()
 - roll-back the changes if possible on error - Jackub

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>