DDF: compare_super_ddf: merge local info of other superblock

DDF: compare_super_ddf: merge local info of other superblock

If a match is found in compare_super_ddf, check the other SB
for local DDF information (VD config records, physical disk data)
which is not available in the current superblock, and add it
if needed.

This is important for the mdmon - when disks are added to a
auto read-only array, they must be present in the DDF structure
in order to guarantee consistent writeback of metadata to all
disks.

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