It's not uncommon to have two access two unrelated memory locations in a
specific order.  At the moment one has to use a memory barrier for this.

However, if the first access was a read and the second used an address
depending on the first one we would have a data dependency and no
barrier would be necessary.

This adds a new interface: dependent_ptr_mb which does exactly this: it
returns a pointer with a data dependency on the supplied value.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>