zvol_disk_open() may spin on CPU

`zvol_disk_open()` waits for up to `zfs_vdev_open_timeout_ms` (1 second by
default) (e.g. if the block device does not exist).  While in this loop,
it calls `schedule_timeout()`.

The problem is that `schedule_timeout()` may not actually cause the
thread to go off-CPU.  Per the "documentation" (comment in the source
code):
```
 * The function behavior depends on the current task state:
 * %TASK_RUNNING - the scheduler is called, but the task does not sleep
 * at all. That happens because sched_submit_work() does nothing for
 * tasks in %TASK_RUNNING state.
```

In my experience, `schedule_timeout()` never sleeps from this code path.
This is especially noticeable if `zfs_vdev_open_timeout_ms` has been
increased from its default.

This commit uses `msleep()` to actually sleep.  Note that this is how it
was before openzfs#7629.

module/os/linux/zfs/vdev_disk.c

@@ -364,10 +364,10 @@ vdev_disk_open(vdev_t *v, uint64_t *psize, uint64_t *max_psize,
 			if (v->vdev_removed)
 				break;
 
-			schedule_timeout(MSEC_TO_TICK(10));
+			msleep(10);
 		} else if (unlikely(PTR_ERR(bdev) == -ERESTARTSYS)) {
 			timeout = MSEC2NSEC(zfs_vdev_open_timeout_ms * 10);
-			continue;
+			msleep(10);
 		} else if (IS_ERR(bdev)) {
 			break;
 		}

module/os/linux/zfs/zvol_os.c

@@ -769,7 +769,7 @@ zvol_open(struct block_device *bdev, fmode_t flag)
 				if ((gethrtime() - start) > timeout)
 					return (SET_ERROR(-ERESTARTSYS));
 
-				schedule_timeout(MSEC_TO_TICK(10));
+				msleep(10);
 				goto retry;
 #endif
 			} else {