padata: Fix wrong usage of rcu_dereference()

A kernel with enabled lockdep complains about the wrong usage of
rcu_dereference() under a rcu_read_lock_bh() protected region.

  ===============================
  [ INFO: suspicious RCU usage. ]
  3.13.0-rc1+ #126 Not tainted
  -------------------------------
  linux/kernel/padata.c:115 suspicious rcu_dereference_check() usage!

  other info that might help us debug this:

  rcu_scheduler_active = 1, debug_locks = 1
  1 lock held by cryptomgr_test/153:
   #0:  (rcu_read_lock_bh){.+....}, at: [<ffffffff8115c235>] padata_do_parallel+0x5/0x270

Fix that by using rcu_dereference_bh() instead.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Acked-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>