xfrm: policy: silence sparse warning in xfrm_policy_unregister_afinfo

xfrm_policy_afinfo is __rcu, use rcu_access_pointer to silence:

net/xfrm/xfrm_policy.c:4152:43: error: incompatible types in comparison expression (different address spaces):
net/xfrm/xfrm_policy.c:4152:43:    struct xfrm_policy_afinfo const [noderef] __rcu *
net/xfrm/xfrm_policy.c:4152:43:    struct xfrm_policy_afinfo const *

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>