scsi: core: Introduce the scsi_cmd_to_rq() function

scsi: core: Introduce the scsi_cmd_to_rq() function

The 'request' member of struct scsi_cmnd is superfluous. The struct request
and struct scsi_cmnd data structures are adjacent and hence the request
pointer can be derived easily from a scsi_cmnd pointer. Introduce a helper
function that performs that conversion in a type-safe way. This patch is
the first step towards removing the request member from struct
scsi_cmnd. Making that change has the following advantages:

 - This is a performance optimization since adding an offset to a pointer
   takes less time than dereferencing a pointer.

 - struct scsi_cmnd becomes smaller.

Link: https://lore.kernel.org/r/20210809230355.8186-2-bvanassche@acm.org
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>