An MBR knows (mbr_lba_self) where it is supposed to go, no

An MBR knows (mbr_lba_self) where it is supposed to go, no
need to pass the value to MBR_write().

Let MBR_write() do the translation from struct mbr to the
struct dos_mbr that will be written to the disk. Thus
eliminating unnecessary struct dos_mbr variables and the
parsing thereof.

No intentional functional change.