block: queue data commands from the flush state machine at the head

We used to insert the data commands following a pre-flush to the head
of the queue until commit 1e82fadfc6b ("blk-mq: do not do head insertions
post-pre-flush commands").  Not doing this seems to cause hangs of
such commands on NFS workloads when exported from file systems with
SATA SSDs.  I have no idea why this would starve these workloads,
but doing a semantic revert of this patch (which looks quite different
due to various other changes) fixes the hangs.

Fixes: 1e82fadfc6b ("blk-mq: do not do head insertions post-pre-flush commands")
Reported-by: Chuck Lever <[email protected]>
Signed-off-by: Christoph Hellwig <[email protected]>
Tested-by: Chuck Lever <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

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