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

... __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.