xen/xenbus: Don't leak memory when unmapping the ring on HVM backend

xen/xenbus: Don't leak memory when unmapping the ring on HVM backend

commit c22fe519e7e2b94ad173e0ea3b89c1a7d8be8d00 upstream.

The commit ccc9d90a9a8b5c4ad7e9708ec41f75ff9e98d61d "xenbus_client:
Extend interface to support multi-page ring" removes the call to
free_xenballooned_pages() in xenbus_unmap_ring_vfree_hvm(), leaking a
page for every shared ring.

Only with backends running in HVM domains were affected.

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>