DDF: use existing locations for primary and secondary DDF structure

DDF: use existing locations for primary and secondary DDF structure

Some RAID BIOSes apparently use hard-coded LBA offsets (presumably
from the end of the disk) for the primary and secondary DDF
structure, ignoring the values given in the DDF anchor. This is
broken BIOS behavior, but it will cause any changes made by MD
(e.g. setting the init_state flag after a full initialization)
to be "forgotten" after the next reboot.

This patch fixes this by using the exiting LBA locations if
available. Verified that this fixes MD+LSI Mega Software RAID
BIOS.

Signed-off-by: Martin Wilck <mwilck@arcor.de>
Signed-off-by: NeilBrown <neilb@suse.de>