net: Handle null dst in rtnl_put_cacheinfo

Need to keep expires time for IPv6 routes in a dump of FIB entries.
Update rtnl_put_cacheinfo to allow dst to be NULL in which case
rta_cacheinfo will only contain non-dst data.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>