[PATCH] Fix on-the-fly switch from cfq i/o scheduler

Don't clear ->elevator_data on exit, if we are switching queues we are
overwriting the data of the new io scheduler.

Signed-off-by: Jens Axboe <axboe@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>