block: BFQ: Move an invariant check

Check bfqq->dispatched for each BFQ queue instead of checking it for an
invalid bfqq pointer.

Fixes: 3e49c1e4 ("block: BFQ: Add several invariant checks")
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20230519220347.3643295-1-bvanassche@acm.org


Signed-off-by: Jens Axboe <axboe@kernel.dk>