virtio_balloon: prevent pfn array overflow

virtio_balloon: prevent pfn array overflow

BugLink: https://bugs.launchpad.net/bugs/1867837
[ Upstream commit 6e9826e77249355c09db6ba41cd3f84e89f4b614 ]

Make sure, at build time, that pfn array is big enough to hold a single
page.  It happens to be true since the PAGE_SHIFT value at the moment is
20, which is 1M - exactly 256 4K balloon pages.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>