[XFRM]: Avoid bogus BUG() when throwing new policy away.

From: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>

When we destory a new policy entry, we need to tell
xfrm_policy_destroy() explicitly that the entry is not
alive yet.

Signed-off-by: David S. Miller <davem@davemloft.net>