xen/blkback: safely unmap purge persistent grants

xen/blkback: safely unmap purge persistent grants

Commit 6f9de5b41f36 ("xen-blkback: safely unmap grants in case they
are still in use") use gnttab_unmap_refs_async() to wait until the
mapped pages are no longer in use before unmapping them, but that
commit missed the persistent case.  Purge persistent pages can't be
unmapped either unless no longer in use.

Signed-off-by: Bob Liu <bob.liu@oracle.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>