padata: Fix pd UAF once and for all

There is a race condition/UAF in padata_reorder that goes back
to the initial commit.  A reference count is taken at the start
of the process in padata_do_parallel, and released at the end in
padata_serial_worker.

This reference count is (and only is) required for padata_replace
to function correctly.  If padata_replace is never called then
there is no issue.

In the function padata_reorder which serves as the core of padata,
as soon as padata is added to queue->serial.list, and the associated
spin lock released, that padata may be processed and the reference
count on pd would go away.

Fix this by getting the next padata before the squeue->serial lock
is released.

In order to make this possible, simplify padata_reorder by only
calling it once the next padata arrives.

Fixes: 16295bec ("padata: Generic parallelization/serialization interface")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>