blk-mq: bitmap tag: fix races in bt_get() function

blk-mq: bitmap tag: fix races in bt_get() function

This update fixes few issues in bt_get() function:

- list_empty(&wait.task_list) check is not protected;

- was_empty check is always true which results in *every* thread
  entering the loop resets bt_wait_state::wait_cnt counter rather
  than every bt->wake_cnt'th thread;

- 'bt_wait_state::wait_cnt' counter update is redundant, since
  it also gets reset in bt_clear_tag() function;

Cc: Christoph Hellwig <[email protected]>
Cc: Ming Lei <[email protected]>
Cc: Jens Axboe <[email protected]>
Signed-off-by: Alexander Gordeev <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>