blk-mq: handle already freed tags gracefully in blk_mq_free_rqs

To simplify further changes allow for double calling blk_mq_free_rqs on
a queue.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
[hch: split out from a larger patch]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20220308055200.735835-3-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>