[IPSEC]: Only set neighbour on top xfrm dst

The neighbour field is only used by dst_confirm which only ever happens on
the top-most xfrm dst.  So it's a waste to duplicate for every other xfrm
dst.  This patch moves its setting out of the loop so that only the top one
gets set.

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