block: make xenbus device id constant

The ids field of the struct xenbus_device_id is constant in <linux/xen/xenbus.h>
so it is worth to make blkfront_ids also constant.

The semantic match that finds this kind of pattern is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@r@
disable decl_init,const_decl_init;
identifier I1, I2, x;
@@
	struct I1 {
	  ...
	  const struct I2 *x;
	  ...
	};
@s@
identifier r.I1, y;
identifier r.x, E;
@@
	struct I1 y = {
	  .x = E,
	};
@c@
identifier r.I2;
identifier s.E;
@@
	const struct I2 E[] = ... ;
@depends on !c@
identifier r.I2;
identifier s.E;
@@
+	const
	struct I2 E[] = ...;
// </smpl>
Signed-off-by: Márton Németh <nm127@freemail.hu>
Cc: Julia Lawall <julia@diku.dk>
Cc: cocci@diku.dk
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

drivers/block/xen-blkfront.c

@@ -1050,7 +1050,7 @@ static const struct block_device_operations xlvbd_block_fops =
 };
 
 
-static struct xenbus_device_id blkfront_ids[] = {
+static const struct xenbus_device_id blkfront_ids[] = {
 	{ "vbd" },
 	{ "" }
 };