scsi: core: Add struct for args to execution functions

Move the SCSI execution functions to use a struct for passing in optional
args. This commit adds the new struct, temporarily converts scsi_execute()
and scsi_execute_req() ands a new helper, scsi_execute_cmd(), which takes
the scsi_exec_args struct.

There should be no change in behavior. We no longer allow users to pass in
any request->rq_flags value, but they were only passing in RQF_PM which we
do support by allowing users to pass in the BLK_MQ_REQ flags used by
blk_mq_alloc_request().

Subsequent commits will convert scsi_execute() and scsi_execute_req() users
to the new helpers then remove scsi_execute() and scsi_execute_req().

Signed-off-by: Mike Christie <michael.christie@oracle.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>