crypto: pcrypt - 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/crypto/pcrypt.c:81 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: [<ffffffff812c8075>] pcrypt_do_parallel.isra.2+0x5/0x200

Fix that by using rcu_dereference_bh() instead.
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Cc: "David S. Miller" <davem@davemloft.net>
Acked-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/pcrypt.c

@@ -78,7 +78,7 @@ static int pcrypt_do_parallel(struct padata_priv *padata, unsigned int *cb_cpu,
 	cpu = *cb_cpu;
 
 	rcu_read_lock_bh();
-	cpumask = rcu_dereference(pcrypt->cb_cpumask);
+	cpumask = rcu_dereference_bh(pcrypt->cb_cpumask);
 	if (cpumask_test_cpu(cpu, cpumask->mask))
 			goto out;