blk-flush: count inflight flush_data requests

The flush state machine use a double list to link all inflight
flush_data requests, to avoid issuing separate post-flushes for
these flush_data requests which shared PREFLUSH.

So we can't reuse rq->queuelist, this is why we need rq->flush.list

In preparation of the next patch that reuse rq->queuelist for flush
state machine, we change the double linked list to unsigned long
counter, which count all inflight flush_data requests.

This is ok since we only need to know if there is any inflight
flush_data request, so unsigned long counter is good.

Signed-off-by: Chengming Zhou <[email protected]>
Reviewed-by: Ming Lei <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

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