Current code uses the specified ring buffer size (either the default of 128
Kbytes or a module parameter specified value) to encompass the one page
ring buffer header plus the actual ring itself.  When the page size is 4K,
carving off one page for the header isn't significant.  But when the page
size is 64K on ARM64, only half of the default 128 Kbytes is left for the
actual ring.  While this doesn't break anything, the smaller ring size
could be a performance bottleneck.

Fix this by applying the VMBUS_RING_SIZE macro to the specified ring buffer
size.  This macro adds a page for the header, and rounds up the size to a
page boundary, using the page size for which the kernel is built.  Use this
new size for subsequent ring buffer calculations.  For example, on ARM64
with 64K page size and the default ring size, this results in the actual
ring being 128 Kbytes, which is intended.

Cc: stable@vger.kernel.org # 5.15.x
Signed-off-by: Michael Kelley <mhklinux@outlook.com>
Link: https://lore.kernel.org/r/20240122170956.496436-1-mhklinux@outlook.comSigned-off-by: Martin K. Petersen <martin.petersen@oracle.com>