padata: Fix list iterator in padata_do_serial()

list_for_each_entry_reverse() assumes that the iterated list is nonempty
and that every list_head is embedded in the same type, but its use in
padata_do_serial() breaks both rules.

This doesn't cause any issues now because padata_priv and padata_list
happen to have their list fields at the same offset, but we really
shouldn't be relying on that.

Fixes: bfde23ce ("padata: unbind parallel jobs from specific CPUs")
Signed-off-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>