md/raid0: fix uninitialized variable bug

If this function fails the callers expect that *private_conf is set to
an ERR_PTR() but that isn't true for the first error path where we can't
allocate "conf".  It leads to some uninitialized variable bugs.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Shaohua Li <shli@fb.com>