blk-mq: fix regression from commit 624dbe47

When the code was collapsed to avoid duplication, the recent patch
for ensuring that a queue is idled before free was dropped, which was
added by commit 19c5d84f

.

Add back the blk_mq_tag_idle(), to ensure we don't leak a reference
to an active queue when it is freed.

Signed-off-by: Jens Axboe <axboe@fb.com>