block: allow disk to have extended device number

Now that disk and partition handlings are mostly unified, it's easy to
allow disk to have extended device number.  This patch makes
add_disk() use extended device number if disk->minors is zero.  Both
sd and ide-disk are updated to use this.

* sd_format_disk_name() is implemented which can generically determine
  the drive name.  This removes disk number restriction stemming from
  limited device names.

* If sd index goes over SD_MAX_DISKS (which can be increased now BTW),
  sd simply doesn't initialize minors letting block layer choose
  extended device number.

* If CONFIG_DEBUG_EXT_DEVT is set, both sd and ide-disk always set
  minors to 0 and use extended device numbers.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

include/linux/genhd.h

@@ -59,6 +59,7 @@ enum {
 };
 
 #define DISK_MAX_PARTS			256
+#define DISK_NAME_LEN			32
 
 #include <linux/major.h>
 #include <linux/device.h>
@@ -140,7 +141,7 @@ struct gendisk {
 	int minors;                     /* maximum number of minors, =1 for
                                          * disks that can't be partitioned. */
 
-	char disk_name[32];		/* name of major driver */
+	char disk_name[DISK_NAME_LEN];	/* name of major driver */
 
 	/* Array of pointers to partitions indexed by partno.
 	 * Protected with matching bdev lock but stat and other