md: fix problems with freeing private data after ->run failure.

If ->run() fails, it can either free the data structures it
allocated, or leave that task to ->free() which will be called
on failures.

However:
  md.c calls ->free() even if ->private_data is NULL, which
     causes problems in some personalities.
  raid0.c frees the data, but doesn't clear ->private_data,
     which will become a problem when we fix md.c

So better fix both these issues at once.

Reported-by: Richard W.M. Jones <rjones@redhat.com>
Fixes: 5aa61f42
URL: https://bugzilla.kernel.org/show_bug.cgi?id=94381


Signed-off-by: NeilBrown <neilb@suse.de>