uio_hv_generic: set size of ring buffer attribute

The original code had ring size as a module parameter, but
then it was made a fixed value.  The code to set the size of
the ring buffer binary file was lost in the transistion.
The size is needed by user mode driver to know the size of
the ring buffer.

Fixes: 37b96a4 ("uio_hv_generic: support sub-channels")
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/uio/uio_hv_generic.c

@@ -171,12 +171,12 @@ static int hv_uio_ring_mmap(struct file *filp, struct kobject *kobj,
 	return 0;
 }
 
-static struct bin_attribute ring_buffer_bin_attr __ro_after_init = {
+static const struct bin_attribute ring_buffer_bin_attr = {
 	.attr = {
 		.name = "ring",
 		.mode = 0600,
-		/* size is set at init time */
 	},
+	.size = 2 * HV_RING_SIZE * PAGE_SIZE,
 	.mmap = hv_uio_ring_mmap,
 };