super-ddf: optimize DDF header search for widely used RAID controllers

super-ddf: optimize DDF header search for widely used RAID controllers

Implemented fallback logic to search the last 32MB of the device
for the DDF header (magic). If found, proceeds to load the DDF metadata
from the located position.

When clearing metadata as required by the mdadm --zero (function Kill),
also erase the last 32MB of data; otherwise, it may result in an
infinite loop.

According to the specification, the Anchor Header should be placed at
the end of the disk. However,some widely used RAID hardware, such as
LSI and PERC, do not position it within the last 512 bytes of the disk.

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