Define the ring buffer size as a constant expression because it should
not depend on the guest page size.

Signed-off-by: Maya Nakamura <m.maya.nakamura@gmail.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Sasha Levin <sashal@kernel.org>