Drivers: hv: util: Specify ring buffer size using Hyper-V page size

VMbus ring buffers are sized based on the 4K page size used by
Hyper-V. The Linux guest page size may not be 4K on all architectures
so use the Hyper-V page size to specify the ring buffer size.

Signed-off-by: Himadri Pandya <himadri18.07@gmail.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>