ibmveth: Automatically enable larger rx buffer pools for larger mtu

Currently, ibmveth maintains several rx buffer pools, which can
be modified through sysfs. By default, pools are not allocated by
default such that jumbo frames cannot be supported without first
activating larger rx buffer pools. This results in failures when attempting
to change the mtu. This patch makes ibmveth automatically allocate
these larger buffer pools when the mtu is changed.

Signed-off-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>