virtio_ballon: change stub of release_pages_by_pfn

virtio_ballon: change stub of release_pages_by_pfn

[ Upstream commit b4d34037329f46ed818d3b0a6e1e23b9c8721f79 ]

and rename it to release_pages_balloon. The function originally takes
arrays of pfns and now it takes pointer to struct virtio_ballon.
This change is necessary to conditionally call adjust_managed_page_count
in the next patch.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>