ip6mr: fix UAF issue in ip6mr_sk_done() when addrconf_init_net() failed

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

ip6mr: fix UAF issue in ip6mr_sk_done() when addrconf_init_net() failed

If the initialization fails in calling addrconf_init_net(), devconf_all is
the pointer that has been released. Then ip6mr_sk_done() is called to
release the net, accessing devconf->mc_forwarding directly causes invalid
pointer access.

The process is as follows:
setup_net()
ops_init()
addrconf_init_net()
all = kmemdup(...) ---> alloc "all"
...
net->ipv6.devconf_all = all;
__addrconf_sysctl_register() ---> failed
...
kfree(all); ---> ipv6.devconf_all invalid
...
ops_exit_list()
...
ip6mr_sk_done()
devconf = net->ipv6.devconf_all;
//devconf is invalid pointer
if (!devconf || !atomic_read(&devconf->mc_forwarding))

The following is the Call Trace information:
BUG: KASAN: use-after-free in ip6mr_sk_done+0x112/0x3a0
Read of size 4 at addr ffff888075508e88 by task ip/14554
Call Trace:
<TASK>
dump_stack_lvl+0x8e/0xd1
print_report+0x155/0x454
kasan_report+0xba/0x1f0
kasan_check_range+0x35/0x1b0
ip6mr_sk_done+0x112/0x3a0
rawv6_close+0x48/0x70
inet_release+0x109/0x230
inet6_release+0x4c/0x70
sock_release+0x87/0x1b0
igmp6_net_exit+0x6b/0x170
ops_exit_list+0xb0/0x170
setup_net+0x7ac/0xbd0
copy_net_ns+0x2e6/0x6b0
create_new_namespaces+0x382/0xa50
unshare_nsproxy_namespaces+0xa6/0x1c0
ksys_unshare+0x3a4/0x7e0
__x64_sys_unshare+0x2d/0x40
do_syscall_64+0x35/0x80
entry_SYSCALL_64_after_hwframe+0x46/0xb0
RIP: 0033:0x7f7963322547

</TASK>
Allocated by task 14554:
kasan_save_stack+0x1e/0x40
kasan_set_track+0x21/0x30
__kasan_kmalloc+0xa1/0xb0
__kmalloc_node_track_caller+0x4a/0xb0
kmemdup+0x28/0x60
addrconf_init_net+0x1be/0x840
ops_init+0xa5/0x410
setup_net+0x5aa/0xbd0
copy_net_ns+0x2e6/0x6b0
create_new_namespaces+0x382/0xa50
unshare_nsproxy_namespaces+0xa6/0x1c0
ksys_unshare+0x3a4/0x7e0
__x64_sys_unshare+0x2d/0x40
do_syscall_64+0x35/0x80
entry_SYSCALL_64_after_hwframe+0x46/0xb0

Freed by task 14554:
kasan_save_stack+0x1e/0x40
kasan_set_track+0x21/0x30
kasan_save_free_info+0x2a/0x40
____kasan_slab_free+0x155/0x1b0
slab_free_freelist_hook+0x11b/0x220
__kmem_cache_free+0xa4/0x360
addrconf_init_net+0x623/0x840
ops_init+0xa5/0x410
setup_net+0x5aa/0xbd0
copy_net_ns+0x2e6/0x6b0
create_new_namespaces+0x382/0xa50
unshare_nsproxy_namespaces+0xa6/0x1c0
ksys_unshare+0x3a4/0x7e0
__x64_sys_unshare+0x2d/0x40
do_syscall_64+0x35/0x80
entry_SYSCALL_64_after_hwframe+0x46/0xb0