block: Simplify blk_mq_dispatch_rq_list() and its callers

block: Simplify blk_mq_dispatch_rq_list() and its callers

The 'nr_budgets' argument of blk_mq_dispatch_rq_list() is either the
number of elements in the 'list' argument or zero. Instead of passing
the number of list elements to blk_mq_dispatch_rq_list(), pass a boolean
argument that indicates whether or not blk_mq_dispatch_rq_list() should
request the block driver for a budget for each request in 'list'.

Remove the code for counting list elements from blk_mq_dispatch_rq_list()
callers where possible. Remove the code that decrements nr_budgets from
blk_mq_dispatch_rq_list() because it is superfluous. Each request that
is processed by blk_mq_dispatch_rq_list() is in one of these two states
if 'get_budget' is false:
* Either the request is on 'list' and the budget for the request has to
  be released from the error path.
* Or the request is not on 'list' and q->mq_ops->queue_rq() has already
  released the budget (ret != BLK_STS_OK) or q->mq_ops->queue_rq() will
  release the budget asynchronously (ret == BLK_STS_OK).

Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: John Garry <john.g.garry@oracle.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20250415205134.3650042-1-bvanassche@acm.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>