block: fix module reference leak in mq-deadline I/O scheduler

block: fix module reference leak in mq-deadline I/O scheduler

During probe, when the block layer registers a request queue, it
defaults to the mq-deadline I/O scheduler if the device is single-queue
and the mq-deadline module is available. To determine availability, the
elevator_set_default() invokes elevator_find_get(), which increments the
module's reference count. However, this reference is never released,
resulting in a module reference leak that prevents the mq-deadline module
from being unloaded.

This patch fixes the issue by ensuring the acquired module reference is
properly released.

Fixes: 1e44bedbc921 ("block: unifying elevator change")
Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250719132722.769536-1-nilay@linux.ibm.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>