vfio: fix expanding DMA area in ppc64le

vfio: fix expanding DMA area in ppc64le

In ppc64le, expanding DMA areas always fail because we cannot remove
a DMA window. As a result, we cannot allocate more than one memseg in
ppc64le. This is because vfio_spapr_dma_mem_map() doesn't unmap all
the mapped DMA before removing the window. This patch fixes this
incorrect behavior.

I also fixed the order of ioctl for unregister and unmap. The ioctl
for unregister sometimes report device busy errors due to the
existence of mapped area.

Signed-off-by: Takeshi Yoshimura <tyos@jp.ibm.com>
Acked-by: David Christensen <drc@linux.vnet.ibm.com>