Drivers: hv: Use kmalloc_array() instead of kmalloc()

Documentation/process/deprecated.rst recommends against the use of
kmalloc with dynamic size calculations due to the risk of overflow and
smaller allocation being made than the caller was expecting.

Replace kmalloc() with kmalloc_array() in hv_common.c to make the
intended allocation size clearer and avoid potential overflow issues.

The number of pages (pgcount) is bounded, so overflow is not a
practical concern here. However, using kmalloc_array() better reflects
the intent to allocate an array and improves consistency with other
allocations.

No functional change intended.

Signed-off-by: Rahul Kumar <rk0006818@gmail.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>