super-ddf: Prevent crash when handling DDF metadata

super-ddf: Prevent crash when handling DDF metadata

A dummy function is defined because availability of ss->update_super is
not always verified.

This fix addresses a crash reported when assembling a RAID array using
mdadm with DDF metadata. For more details, see the discussion at:
https://lore.kernel.org/all/
CALHdMH30LuxR4tz9jP2ykDaDJtZ3P7L3LrZ+9e4Fq=Q6NwSM=Q@mail.gmail.com/

The discussion centers on an issue with mdadm where attempting to
assemble a RAID array caused a null pointer dereference. The problem
was traced to a missing update_super() function in super-ddf.c, which
led to a crash in Assemble.c.

Signed-off-by: lilinzhe <llz@antiy.cn>