ibmveth: Use lighter weight read memory barrier in ibmveth_poll

We want to order the read in ibmveth_rxq_pending_buffer and the read of
ibmveth_rxq_buffer_valid which are both cacheable memory. smp_rmb() is good
enough for this.

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>