KMSAN: uninit-value in __dev_mc_add

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    eb2e67596de2 kmsan: minor code cleanup
git tree:       https://github.com/google/kmsan.git/master
console output: https://syzkaller.appspot.com/x/log.txt?x=136e3b11400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=94a9ed72288f7fef
dashboard link: https://syzkaller.appspot.com/bug?extid=001516d86dbe88862cec
compiler:       clang version 8.0.0 (trunk 339414)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+001516d86dbe88862cec@xxxxxxxxxxxxxxxxxxxxxxxxx

==================================================================
BUG: KMSAN: uninit-value in memcmp+0x11d/0x180 lib/string.c:863
CPU: 0 PID: 14033 Comm: kworker/0:3 Not tainted 4.19.0-rc4+ #58
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Workqueue: ipv6_addrconf addrconf_dad_work
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x2f6/0x430 lib/dump_stack.c:113
 kmsan_report+0x183/0x2b0 mm/kmsan/kmsan.c:917
 __msan_warning+0x70/0xc0 mm/kmsan/kmsan_instr.c:500
 memcmp+0x11d/0x180 lib/string.c:863
 __hw_addr_add_ex net/core/dev_addr_lists.c:61 [inline]
 __dev_mc_add+0x215/0x8c0 net/core/dev_addr_lists.c:670
 dev_mc_add+0x6d/0x80 net/core/dev_addr_lists.c:687
 igmp6_group_added+0x2c8/0xaa0 net/ipv6/mcast.c:676
 __ipv6_dev_mc_inc+0xf4b/0x1270 net/ipv6/mcast.c:934
 ipv6_dev_mc_inc+0x70/0x80 net/ipv6/mcast.c:941
 addrconf_join_solict net/ipv6/addrconf.c:2098 [inline]
 addrconf_dad_begin net/ipv6/addrconf.c:3879 [inline]
 addrconf_dad_work+0x437/0x29b0 net/ipv6/addrconf.c:4006
 process_one_work+0x197f/0x2490 kernel/workqueue.c:2153
 worker_thread+0x1f6a/0x2b00 kernel/workqueue.c:2296
 kthread+0x59c/0x5d0 kernel/kthread.c:247
 ret_from_fork+0x35/0x40 arch/x86/entry/entry_64.S:416

Local variable description: ----buf@igmp6_group_added
Variable was created at:
 igmp6_group_added+0x46/0xaa0 net/ipv6/mcast.c:664
 __ipv6_dev_mc_inc+0xf4b/0x1270 net/ipv6/mcast.c:934
==================================================================


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