padata: fix null pointer deref of pd->pinst

The 4.19 backport dc34710 ("padata: Remove broken queue flushing")
removed padata_alloc_pd()'s assignment to pd->pinst, resulting in:

    Unable to handle kernel NULL pointer dereference ...
    ...
    pc : padata_reorder+0x144/0x2e0
    ...
    Call trace:
     padata_reorder+0x144/0x2e0
     padata_do_serial+0xc8/0x128
     pcrypt_aead_enc+0x60/0x70 [pcrypt]
     padata_parallel_worker+0xd8/0x138
     process_one_work+0x1bc/0x4b8
     worker_thread+0x164/0x580
     kthread+0x134/0x138
     ret_from_fork+0x10/0x18

This happened because the backport was based on an enhancement that
moved this assignment but isn't in 4.19:

  bfde23c ("padata: unbind parallel jobs from specific CPUs")

Simply restore the assignment to fix the crash.

Fixes: dc34710 ("padata: Remove broken queue flushing")
Reported-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Sasha Levin <sashal@kernel.org>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: linux-kernel@vger.kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

kernel/padata.c

@@ -510,6 +510,7 @@ static struct parallel_data *padata_alloc_pd(struct padata_instance *pinst,
 	atomic_set(&pd->seq_nr, -1);
 	atomic_set(&pd->reorder_objects, 0);
 	atomic_set(&pd->refcnt, 1);
+	pd->pinst = pinst;
 	spin_lock_init(&pd->lock);
 
 	return pd;