[XFRM]: Fix leak of expired xfrm_states

The xfrm_timer calls __xfrm_state_delete, which drops the final reference
manually without triggering destruction of the state. Change it to use
xfrm_state_put to add the state to the gc list when we're dropping the
last reference. The timer function may still continue to use the state
safely since the final destruction does a del_timer_sync().

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>