Copy code from scsi_read_write() as mfi_build_syspd_cdb() to build SCSI
command properly. Without this change, mfi(4) always sends 10 byte READ
and WRITE commands, which will cause data corruption when device is
larger than 2^32 sectors.

PR: kern/173291
Submitted by: Steven Hartland <steven.hartland multiplay.co.uk>
Reviewed by: mav
MFC after: 2 weeks