ipv4: xfrm: Eliminate ->rt_src reference in policy code.

Rearrange xfrm4_dst_lookup() so that it works by calling a helper
function __xfrm_dst_lookup() that takes an explicit flow key storage
area as an argument.

Use this new helper in xfrm4_get_saddr() so we can fetch the selected
source address from the flow instead of from rt->rt_src

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