ibmveth: Add optional flush of rx buffer

On some machines we can improve the bandwidth by ensuring rx buffers are
not in the cache. Add a module option that is disabled by default that flushes
rx buffers on insertion.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Santiago Leon <santil@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>