net: ipv6: ping: Use socket mark in routing lookup

[net-next commit bf439b3154ce49d81a79b14f9fab18af99018ae2]

Change-Id: I8356e9132088c75d4510021c6e4c2641d772087a
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>