ipv4: fix ipsec forward performance regression

There is bug in commit 5e2b61f(ipv4: Remove flowi from struct rtable).
It makes xfrm4_fill_dst() modify wrong data structure.

Signed-off-by: Zheng Yan <zheng.z.yan@intel.com>
Reported-by: Kim Phillips <kim.phillips@freescale.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>