Input: hyperv-keyboard: Use VMBUS_RING_SIZE() for ringbuffer sizes

For a Hyper-V vmbus, the size of the ringbuffer has two requirements:

1) it has to take one PAGE_SIZE for the header

2) it has to be PAGE_SIZE aligned so that double-mapping can work

VMBUS_RING_SIZE() could calculate a correct ringbuffer size which
fulfills both requirements, therefore use it to make sure vmbus work
when PAGE_SIZE != HV_HYP_PAGE_SIZE (4K).

Note that since the argument for VMBUS_RING_SIZE() is the size of
payload (data part), so it will be minus 4k (the size of header when
PAGE_SIZE = 4k) than the original value to keep the ringbuffer total
size unchanged when PAGE_SIZE = 4k.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Cc: Michael Kelley <mikelley@microsoft.com>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/20200916034817.30282-9-boqun.feng@gmail.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>