[SCSI] scsi: sd, sr, st, and scsi_lib all fail to copy cmd_len to new cmd

This fixes an issue in scsi command initialization from a request
where sd, sr, st, and scsi_lib all fail to copy the request's
cmd_len to the scsi command's cmd_len field.

Signed-off-by: Timothy Thelin <timothy.thelin@wdc.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>