xfrm: do not call rcu_read_unlock when afinfo is NULL in xfrm_get_tos

When xfrm_policy_get_afinfo returns NULL, it will not hold rcu
read lock. In this case, rcu_read_unlock should not be called
in xfrm_get_tos, just like other places where it's calling
xfrm_policy_get_afinfo.

Fixes: f5e2bb4f ("xfrm: policy: xfrm_get_tos cannot fail")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>