xfrm: fix a read lock imbalance in make_blackhole

if xfrm_policy_get_afinfo returns 0, it has already released the read
lock, xfrm_policy_put_afinfo should not be called again.

Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>