block: add function call to switch the IO scheduler from a driver

Currently drivers must do an elevator_exit() + elevator_init()
to switch IO schedulers. There are a few problems with this:

- Since commit 1abec4fdbb,
  elevator_init() requires a zeroed out q->elevator
  pointer. The two existing in-kernel users don't do that.

- It will only work at initialization time, since using the
  above two-staged construct does not properly quisce the queue.

So add elevator_change() which takes care of this, and convert
the elv_iosched_store() sysfs interface to use this helper as well.

Reported-by: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>
Reported-by: Kevin Vigor <kevin@vigor.nu>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

include/linux/elevator.h

@@ -136,6 +136,7 @@ extern ssize_t elv_iosched_store(struct request_queue *, const char *, size_t);
 
 extern int elevator_init(struct request_queue *, char *);
 extern void elevator_exit(struct elevator_queue *);
+extern int elevator_change(struct request_queue *, const char *);
 extern int elv_rq_merge_ok(struct request *, struct bio *);
 
 /*