Replace rwlock on xfrm_policy_afinfo with rcu

xfrm_policy_afinfo is read mosly data structure.
Write on xfrm_policy_afinfo is done only at the
time of configuration.
So rwlocks can be safely replaced with RCU.

RCUs usage optimizes the performance.

Signed-off-by: Priyanka Jain <Priyanka.Jain@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>