Drivers: hv: hv_balloon: report offline pages as being used

When hot-added memory pages are not brought online or when some memory blocks
are sent offline the subsequent ballooning process kills the guest with OOM
killer. This happens as we don't report these pages as neither used nor free
and apparently host algorithm considers them as being unused. Keep track of
all online/offline operations and report all currently offline pages as being
used so host won't try to balloon them out.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>