net: Fix skb_update_prio RCU usage.

Change function rcu_dereference to rcu_dereference_bh to avoid warning

[ INFO: suspicious RCU usage. ]
-------------------------------
net/core/dev.c:2459 suspicious rcu_dereference_check() usage!

because we are locking with

rcu_read_lock_bh();

in function dev_queue_xmit(struct sk_buff *skb)

Signed-off-by: Igor Maravic <igorm@etf.rs>
Signed-off-by: David S. Miller <davem@davemloft.net>