skbuff: revert "skbuff: remove some unnecessary operation in skb_segment_list()"

the commit 1ddc3229 ("skbuff: remove some unnecessary operation
in skb_segment_list()") introduces an issue very similar to the
one already fixed by commit 53475c5d ("net: fix use-after-free when
UDP GRO with shared fraglist").

If the GSO skb goes though skb_clone() and pskb_expand_head() before
entering skb_segment_list(), the latter  will unshare the frag_list
skbs and will release the old list. With the reverted commit in place,
when skb_segment_list() completes, skb->next points to the just
released list, and later on the kernel will hit UaF.

Note that since commit e0e3070a ("udp: properly complete L4 GRO
over UDP tunnel packet") the critical scenario can be reproduced also
receiving UDP over vxlan traffic with:

NIC (NETIF_F_GRO_FRAGLIST enabled) -> vxlan -> UDP sink

Attaching a packet socket to the NIC will cause skb_clone() and the
tunnel decapsulation will call pskb_expand_head().

Fixes: 1ddc3229 ("skbuff: remove some unnecessary operation in skb_segment_list()")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>