net: ipv4: fix listify ip_rcv_finish in case of forwarding

In commit 5fa12739 ("net: ipv4: listify ip_rcv_finish") calling
dst_input(skb) was split-out.  The ip_sublist_rcv_finish() just calls
dst_input(skb) in a loop.

The problem is that ip_sublist_rcv_finish() forgot to remove the SKB
from the list before invoking dst_input().  Further more we need to
clear skb->next as other parts of the network stack use another kind
of SKB lists for xmit_more (see dev_hard_start_xmit).

A crash occurs if e.g. dst_input() invoke ip_forward(), which calls
dst_output()/ip_output() that eventually calls __dev_queue_xmit() +
sch_direct_xmit(), and a crash occurs in validate_xmit_skb_list().

This patch only fixes the crash, but there is a huge potential for
a performance boost if we can pass an SKB-list through to ip_forward.

Fixes: 5fa12739

 ("net: ipv4: listify ip_rcv_finish")
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>