virtio_ring: Update weak barriers to use dma_wmb/rmb

This change makes it so that instead of using smp_wmb/rmb which varies
depending on the kernel configuration we can can use dma_wmb/rmb which for
most architectures should be equal to or slightly more strict than
smp_wmb/rmb.

The advantage to this is that these barriers are available to uniprocessor
builds as well so the performance should improve under such a
configuration.

Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>