[IPSEC]: Store xfrm states in security path directly

As it is xfrm_input first collects a list of xfrm states on the stack
before storing them in the packet's security path just before it
returns.  For async crypto, this construction presents an obstacle
since we may need to leave the loop after each transform.

In fact, it's much easier to just skip the stack completely and always
store to the security path.  This is proven by the fact that this
patch actually shrinks the code.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>