block: Avoid that sparse complains about context imbalance in __wbt_wait()

This patch does not change any functionality.

Fixes: e34cbd30 ("blk-wbt: add general throttling mechanism")
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Jens Axboe <axboe@fb.com>