scsi_dh_hp_sw: switch to scsi_execute_req_flags()

Switch to scsi_execute_req_flags() instead of using the block interface
directly.  This will set REQ_QUIET and REQ_PREEMPT, but this is okay as
we're evaluating the errors anyway and should be able to send the command
even if the device is quiesced.

Signed-off-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Jens Axboe <axboe@fb.com>