[PATCH] fix try_module_get race in elevator_find

This patch removes try_module_get race in elevator_find.
try_module_get should always be called with the spinlock protecting
what the module init/cleanup routines register/unregister to held. In
the case of elevators, we should be holding elv_list to avoid it going
away between spin_unlock_irq and try_module_get.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jens Axboe <axboe@suse.de>