net: dst: provide accessor function to dst->xfrm

dst->xfrm is conditionally defined.  Provide accessor funtion that
is always available.

Signed-off-by: Vlad Yasevich <vyasevich@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>