vmbus: replace modulus operation with subtraction

Takes less clock cycles to check for ring wrap and subtract than to
do a modulus instruction.

Signed-off-by: Stephen Hemminger <[e-mail]>
Signed-off-by: K. Y. Srinivasan <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/hv/ring_buffer.c

···
 	u32 next = ring_info->ring_buffer->read_index;
 
 	next += offset;
-	next %= ring_info->ring_datasize;
+	if (next >= ring_info->ring_datasize)
+		next -= ring_info->ring_datasize;
 
 	return next;
 }
···
 	memcpy(dest, ring_buffer + start_read_offset, destlen);
 
 	start_read_offset += destlen;
-	start_read_offset %= ring_buffer_size;
+	if (start_read_offset >= ring_buffer_size)
+		start_read_offset -= ring_buffer_size;
 
 	return start_read_offset;
 }
···
 	memcpy(ring_buffer + start_write_offset, src, srclen);
 
 	start_write_offset += srclen;
-	start_write_offset %= ring_buffer_size;
+	if (start_write_offset >= ring_buffer_size)
+		start_write_offset -= ring_buffer_size;
 
 	return start_write_offset;
 }