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: NMarcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: NVlad Buslov <vladbu@mellanox.com>
Reviewed-by: NMarcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>