xfrm/xfrm_policy: fix dst dev null pointer dereference in collect_md mode

In decode_session{4,6} there is a possibility that the skb dst dev is NULL,
e,g, with tunnel collect_md mode, which will cause kernel crash.
Here is what the code path looks like, for GRE:

- ip6gre_tunnel_xmit
  - ip6gre_xmit_ipv6
    - __gre6_xmit
      - ip6_tnl_xmit
        - if skb->len - t->tun_hlen - eth_hlen > mtu; return -EMSGSIZE
    - icmpv6_send
      - icmpv6_route_lookup
        - xfrm_decode_session_reverse
          - decode_session4
            - oif = skb_dst(skb)->dev->ifindex; <-- here
          - decode_session6
            - oif = skb_dst(skb)->dev->ifindex; <-- here

The reason is __metadata_dst_init() init dst->dev to NULL by default.
We could not fix it in __metadata_dst_init() as there is no dev supplied.
On the other hand, the skb_dst(skb)->dev is actually not needed as we
called decode_session{4,6} via xfrm_decode_session_reverse(), so oif is not
used by: fl4->flowi4_oif = reverse ? skb->skb_iif : oif;

So make a dst dev check here should be clean and safe.

v4: No changes.

v3: No changes.

v2: fix the issue in decode_session{4,6} instead of updating shared dst dev
in {ip_md, ip6}_tunnel_xmit.

Fixes: 8d79266b ("ip6_tunnel: add collect_md mode to IPv6 tunnels")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Tested-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>