Add warning for zfs_vdev_elevator option removal

Originally the zfs_vdev_elevator module option was added as a
convenience so the requested elevator would be automatically set
on the underlying block devices.  At the time this was simple
because the kernel provided an API function which did exactly this.

This API was then removed in the Linux 4.12 kernel which prompted
us to add compatibly code to set the elevator via a usermodehelper.
While well intentioned this introduced a bug which could cause a
system hang, that issue was subsequently fixed by commit 2a0d418.

In order to avoid future bugs in this area, and to simplify the code,
this functionality is being deprecated.  A console warning has been
added to notify any existing consumers and the documentation updated
accordingly.  This option will remain for the lifetime of the 0.8.x
series for compatibility but if planned to be phased out of master.

Signed-off-by: Brian Behlendorf <[email protected]>
Issue openzfs#8664
Issue openzfs#9317

man/man5/zfs-module-parameters.5

@@ -3188,8 +3188,10 @@ Default value: \fB32,768\fR.
 \fBzfs_vdev_scheduler\fR (charp)
 .ad
 .RS 12n
-Set the Linux I/O scheduler on whole disk vdevs to this scheduler. Valid options
-are noop, cfq, bfq & deadline
+Set the Linux I/O scheduler on whole disk vdevs to this scheduler.  This
+option has been deprecated and will be removed in a future release.  The
+standard \fB/sys/block/<block>/queue/scheduler\fR interface should be used
+to set a block devices scheduler.
 .sp
 Default value: \fBnoop\fR.
 .RE

module/os/linux/zfs/vdev_disk.c

@@ -930,7 +930,14 @@ param_set_vdev_scheduler(const char *val, zfs_kernel_param_t *kp)
 		mutex_exit(&spa_namespace_lock);
 	}
 
-	return (param_set_charp(val, kp));
+
+	int error = param_set_charp(val, kp);
+	if (error == 0) {
+		printk(KERN_INFO "The 'zfs_vdev_scheduler' module option "
+		    "will be removed in a future release.\n");
+	}
+
+	return (error);
 }
 
 vdev_ops_t vdev_disk_ops = {