PCI: hv: Fix ring buffer size calculation

For a physical PCI device that is passed through to a Hyper-V guest VM,
current code specifies the VMBus ring buffer size as 4 pages.  But this
is an inappropriate dependency, since the amount of ring buffer space
needed is unrelated to PAGE_SIZE. For example, on x86 the ring buffer
size ends up as 16 Kbytes, while on ARM64 with 64 Kbyte pages, the ring
size bloats to 256 Kbytes. The ring buffer for PCI pass-thru devices
is used for only a few messages during device setup and removal, so any
space above a few Kbytes is wasted.

Fix this by declaring the ring buffer size to be a fixed 16 Kbytes.
Furthermore, use the VMBUS_RING_SIZE() macro so that the ring buffer
header is properly accounted for, and so the size is rounded up to a
page boundary, using the page size for which the kernel is built. While
w/64 Kbyte pages this results in a 64 Kbyte ring buffer header plus a
64 Kbyte ring buffer, that's the smallest possible with that page size.
It's still 128 Kbytes better than the current code.

Link: https://lore.kernel.org/linux-pci/20240216202240.251818-1-mhklinux@outlook.com
Signed-off-by: Michael Kelley <mhklinux@outlook.com>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Reviewed-by: Ilpo Jarvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Long Li <longli@microsoft.com>
Cc: <stable@vger.kernel.org> # 5.15.x