sd: don't use scsi_setup_blk_pc_cmnd for flush requests

sd: don't use scsi_setup_blk_pc_cmnd for flush requests

Simplify handling of flush requests by setting up the command directly
instead of initializing request fields and then calling
scsi_setup_blk_pc_cmnd to propagate the information into the command.

Also rename scsi_setup_flush_cmnd to sd_setup_flush_cmnd for consistency.

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