driver: fix unsigned long overflow in leave_hypervisor

driver: fix unsigned long overflow in leave_hypervisor

When shutting down the hypervisor, in the leave_hypervisor
function, the Linux driver touches every hypervisor page, to
ensure all pages are mapped. However, the current implementation
assumes hv_core_and_percpu_size is aligned to PAGE_SIZE. This may
not be the case, if PAGE_SIZE is different on the hypervisor side.

This can cause an unsigned long overflow, leading to an infinite
loop of touching successive pages starting from hypervisor_mem.
The loop will be broken as soon as Linux tries to touch an invalid
page, leading to a kernel crash.

Signed-off-by: Antonios Motakis <antonios.motakis@huawei.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>