blk-throttle: don't take carryover for prioritized processing of metadata

blk-throttle: don't take carryover for prioritized processing of metadata

Commit 29390bb5661d ("blk-throttle: support prioritized processing of metadata")
takes bytes/ios carryover for prioritized processing of metadata. Turns out
we can support it by charging it directly without trimming slice, and the
result is same with carryover.

Cc: Tejun Heo <tj@kernel.org>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20250305043123.3938491-3-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>