padata: Always leave BHs disabled when running ->parallel()

In the Linux kernel, the following vulnerability has been resolved:

padata: Always leave BHs disabled when running ->parallel()

A deadlock can happen when an overloaded system runs ->parallel() in the
context of the current task:

padata_do_parallel
->parallel()
pcrypt_aead_enc/dec
padata_do_serial
spin_lock(&reorder->lock) // BHs still enabled
<interrupt>
...
__do_softirq
...
padata_do_serial
spin_lock(&reorder->lock)

It's a bug for BHs to be on in _do_serial as Steffen points out, so
ensure they're off in the "current task" case like they are in
padata_parallel_worker to avoid this situation.