cfq-iosched: fix the setting of IOPS mode on SSDs

A previous commit wanted to make CFQ default to IOPS mode on
non-rotational storage, however it did so when the queue was
initialized and the non-rotational flag is only set later on
in the probe.

Add an elevator hook that gets called off the add_disk() path,
at that point we know that feature probing has finished, and
we can reliably check for the various flags that drivers can
set.

Fixes: 41c0126b ("block: Make CFQ default to IOPS mode on SSDs")
Tested-by: Romain Francoise <romain@orebokech.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/block/elevator.c b/block/elevator.c
index 59794d0..5f04527 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -810,6 +810,8 @@
 		}
 		kobject_uevent(&e->kobj, KOBJ_ADD);
 		e->registered = 1;
+		if (e->type->ops.elevator_registered_fn)
+			e->type->ops.elevator_registered_fn(q);
 	}
 	return error;
 }