net: sched: fix unprotected access to rcu cookie pointer

Fix action attribute size calculation function to take rcu read lock and
access act_cookie pointer with rcu dereference.

Fixes: eec94fdb ("net: sched: use rcu for action cookie update")
Reported-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Vlad Buslov <vladbu@mellanox.com>
Reviewed-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>