ibmveth: change rx buffer default allocation for CMO

This patch enables 64k rx buffer pools by default.  If Cooperative
Memory Overcommitment (CMO) is enabled, the number of 64k buffers
is reduced to save memory.

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