ping: fix the dif and sdif check in ping_lookup

When 'ping' changes to use PING socket instead of RAW socket by:

   # sysctl -w net.ipv4.ping_group_range="0 100"

There is another regression caused when matching sk_bound_dev_if
and dif, RAW socket is using inet_iif() while PING socket lookup
is using skb->dev->ifindex, the cmd below fails due to this:

  # ip link add dummy0 type dummy
  # ip link set dummy0 up
  # ip addr add 192.168.111.1/24 dev dummy0
  # ping -I dummy0 192.168.111.1 -c1

The issue was also reported on:

  https://github.com/iputils/iputils/issues/104

But fixed in iputils in a wrong way by not binding to device when
destination IP is on device, and it will cause some of kselftests
to fail, as Jianlin noticed.

This patch is to use inet(6)_iif and inet(6)_sdif to get dif and
sdif for PING socket, and keep consistent with RAW socket.

Fixes: c319b4d7

 ("net: ipv4: add IPPROTO_ICMP socket kind")
Reported-by: Jianlin Shi <jishi@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>