um: Use real DMA barriers

When we have virtio enabled, we must have real barriers since we
may be running on an SMP machine (quite likely are, in fact), so
the other process can be on another CPU.

Since in any other case we don't really use DMA barriers, remove
their override completely so real barriers will get used. In the
future we might need them for other cases as well.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>