nvmet: don't use BLK_MQ_REQ_NOWAIT for passthru

nvmet: don't use BLK_MQ_REQ_NOWAIT for passthru

By default, we set the passthru request allocation flag such that it
returns the error in the following code path and we fail the I/O when
BLK_MQ_REQ_NOWAIT is used for request allocation :-

nvme_alloc_request()
 blk_mq_alloc_request()
  blk_mq_queue_enter()
   if (flag & BLK_MQ_REQ_NOWAIT)
        return -EBUSY; <-- return if busy.

On some controllers using BLK_MQ_REQ_NOWAIT ends up in I/O error where
the controller is perfectly healthy and not in a degraded state.

Block layer request allocation does allow us to wait instead of
immediately returning the error when we BLK_MQ_REQ_NOWAIT flag is not
used. This has shown to fix the I/O error problem reported under
heavy random write workload.

Remove the BLK_MQ_REQ_NOWAIT parameter for passthru request allocation
which resolves this issue.

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>