syzbot has bisected this bug to:

commit 58956317c8de52009d1a38a721474c24aef74fe7
Author: David Ahern [off-list ref]
Date:   Fri Dec 7 20:24:57 2018 +0000

     neighbor: Improve garbage collection

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=1328a1e2e00000
start commit:   addb0679 Merge git://git.kernel.org/pub/scm/linux/kernel/g..
git tree:       net-next
final crash:    https://syzkaller.appspot.com/x/report.txt?x=10a8a1e2e00000
console output: https://syzkaller.appspot.com/x/log.txt?x=1728a1e2e00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=d9655b05acfc97ff
dashboard link: https://syzkaller.appspot.com/bug?extid=6a3c02010a025ac7b7cf
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=13317705400000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=177bb01b400000

Reported-by: syzbot+6a3c02010a025ac7b7cf@syzkaller.appspotmail.com
Fixes: 58956317c8de ("neighbor: Improve garbage collection")

For information about bisection process see: https://goo.gl/tpsmEJ#bisection