scsi_dh_alua: switch to scsi_execute_req_flags()

All commands are issued synchronously, so no need to open-code
scsi_execute_req_flags() anymore. And we can get rid of the
static sense code structure element. scsi_execute_req_flags()
will be setting REQ_QUIET and REQ_PREEMPT, but that is
perfectly fine as we're evaluating and logging any errors
ourselves and we really need to send the command even if
the device is quiesced.

Reviewed-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ewan Milne <emilne@redhat.com>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>