hv_uio_generic: map ringbuffer phys addr

The ring buffer is contiguous IOVA and is mapped via phys addr
for sysfs file. Use same method for the UIO mapping.

Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>