rpl: Fix use-after-free in rpl_do_srh_inline().

In the Linux kernel, the following vulnerability has been resolved:

rpl: Fix use-after-free in rpl_do_srh_inline().

Running lwt_dst_cache_ref_loop.sh in selftest with KASAN triggers
the splat below [0].

rpl_do_srh_inline() fetches ipv6_hdr(skb) and accesses it after
skb_cow_head(), which is illegal as the header could be freed then.

Let's fix it by making oldhdr to a local struct instead of a pointer.

[0]:
[root@fedora net]# ./lwt_dst_cache_ref_loop.sh
...
TEST: rpl (input)
[ 57.631529] ==================================================================
BUG: KASAN: slab-use-after-free in rpl_do_srh_inline.isra.0 (net/ipv6/rpl_iptunnel.c:174)
Read of size 40 at addr ffff888122bf96d8 by task ping6/1543

CPU: 50 UID: 0 PID: 1543 Comm: ping6 Not tainted 6.16.0-rc5-01302-gfadd1e6231b1 #23 PREEMPT(voluntary)
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
Call Trace:
<IRQ>
dump_stack_lvl (lib/dump_stack.c:122)
print_report (mm/kasan/report.c:409 mm/kasan/report.c:521)
kasan_report (mm/kasan/report.c:221 mm/kasan/report.c:636)
kasan_check_range (mm/kasan/generic.c:175 (discriminator 1) mm/kasan/generic.c:189 (discriminator 1))
__asan_memmove (mm/kasan/shadow.c:94 (discriminator 2))
rpl_do_srh_inline.isra.0 (net/ipv6/rpl_iptunnel.c:174)
rpl_input (net/ipv6/rpl_iptunnel.c:201 net/ipv6/rpl_iptunnel.c:282)
lwtunnel_input (net/core/lwtunnel.c:459)
ipv6_rcv (./include/net/dst.h:471 (discriminator 1) ./include/net/dst.h:469 (discriminator 1) net/ipv6/ip6_input.c:79 (discriminator 1) ./include/linux/netfilter.h:317 (discriminator 1) ./include/linux/netfilter.h:311 (discriminator 1) net/ipv6/ip6_input.c:311 (discriminator 1))
__netif_receive_skb_one_core (net/core/dev.c:5967)
process_backlog (./include/linux/rcupdate.h:869 net/core/dev.c:6440)
__napi_poll.constprop.0 (net/core/dev.c:7452)
net_rx_action (net/core/dev.c:7518 net/core/dev.c:7643)
handle_softirqs (kernel/softirq.c:579)
do_softirq (kernel/softirq.c:480 (discriminator 20))
</IRQ>
<TASK>
__local_bh_enable_ip (kernel/softirq.c:407)
__dev_queue_xmit (net/core/dev.c:4740)
ip6_finish_output2 (./include/linux/netdevice.h:3358 ./include/net/neighbour.h:526 ./include/net/neighbour.h:540 net/ipv6/ip6_output.c:141)
ip6_finish_output (net/ipv6/ip6_output.c:215 net/ipv6/ip6_output.c:226)
ip6_output (./include/linux/netfilter.h:306 net/ipv6/ip6_output.c:248)
ip6_send_skb (net/ipv6/ip6_output.c:1983)
rawv6_sendmsg (net/ipv6/raw.c:588 net/ipv6/raw.c:918)
__sys_sendto (net/socket.c:714 (discriminator 1) net/socket.c:729 (discriminator 1) net/socket.c:2228 (discriminator 1))
__x64_sys_sendto (net/socket.c:2231)
do_syscall_64 (arch/x86/entry/syscall_64.c:63 (discriminator 1) arch/x86/entry/syscall_64.c:94 (discriminator 1))
entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)
RIP: 0033:0x7f68cffb2a06
Code: 5d e8 41 8b 93 08 03 00 00 59 5e 48 83 f8 fc 75 19 83 e2 39 83 fa 08 75 11 e8 26 ff ff ff 66 0f 1f 44 00 00 48 8b 45 10 0f 05 <48> 8b 5d f8 c9 c3 0f 1f 40 00 f3 0f 1e fa 55 48 89 e5 48 83 ec 08
RSP: 002b:00007ffefb7c53d0 EFLAGS: 00000202 ORIG_RAX: 000000000000002c
RAX: ffffffffffffffda RBX: 0000564cd69f10a0 RCX: 00007f68cffb2a06
RDX: 0000000000000040 RSI: 0000564cd69f10a4 RDI: 0000000000000003
RBP: 00007ffefb7c53f0 R08: 0000564cd6a032ac R09: 000000000000001c
R10: 0000000000000000 R11: 0000000000000202 R12: 0000564cd69f10a4
R13: 0000000000000040 R14: 00007ffefb7c66e0 R15: 0000564cd69f10a0
</TASK>

Allocated by task 1543:
kasan_save_stack (mm/kasan/common.c:48)
kasan_save_track (mm/kasan/common.c:60 (discriminator 1) mm/kasan/common.c:69 (discriminator 1))
__kasan_slab_alloc (mm/kasan/common.c:319 mm/kasan/common.c:345)
kmem_cache_alloc_node_noprof (./include/linux/kasan.h:250 mm/slub.c:4148 mm/slub.c:4197 mm/slub.c:4249)
kmalloc_reserve (net/core/skbuff.c:581 (discriminator 88))
__alloc_skb (net/core/skbuff.c:669)
__ip6_append_data (net/ipv6/ip6_output.c:1672 (discriminator 1))
ip6_
---truncated---