block: make xenbus device id constant

From: Márton Németh <[email protected]>

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>

---

diff -u -p a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
@@ -1050,7 +1050,7 @@ static const struct block_device_operati
 };
 
 
-static struct xenbus_device_id blkfront_ids[] = {
+static const struct xenbus_device_id blkfront_ids[] = {
 	{ "vbd" },
 	{ "" }
 };