rbd: default to single-major device number scheme

It's been 3.5 years, let's turn it on by default.  Support in rbd(8)
utility goes back to pre-firefly, "rbd map" has been loading the module
with single_major=Y ever since.  However, if the module is already
loaded (whether by hand or at boot time), we end up with single_major=N.
Also, some people don't install rbd(8) and use the sysfs interface
directly.

(With single-major=N, a major number is consumed for every mapping,
imposing a limit of ~240 rbd images per host.  single-major=Y allows
mapping thousands of rbd images on a single machine.)

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jason Dillaman <dillaman@redhat.com>

drivers/block/rbd.c

@@ -449,12 +449,11 @@ static DEFINE_IDA(rbd_dev_id_ida);
 static struct workqueue_struct *rbd_wq;
 
 /*
- * Default to false for now, as single-major requires >= 0.75 version of
- * userspace rbd utility.
+ * single-major requires >= 0.75 version of userspace rbd utility.
  */
-static bool single_major = false;
+static bool single_major = true;
 module_param(single_major, bool, S_IRUGO);
-MODULE_PARM_DESC(single_major, "Use a single major number for all rbd devices (default: false)");
+MODULE_PARM_DESC(single_major, "Use a single major number for all rbd devices (default: true)");
 
 static int rbd_img_request_submit(struct rbd_img_request *img_request);