ipv6: fix null pointer dereference in __ip6addrlbl_add

ipv6: fix null pointer dereference in __ip6addrlbl_add

Commit d2c5b6e33350be0e68749c9538c1ac409c57d937 ("hlist: drop
the node parameter from iterators") changed the behavior of
hlist_for_each_entry_safe to leave the p argument NULL.

Fix this up by tracking the last argument.

Reported-by: Michele Baldessari <michele@acksyn.org>
Cc: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
Cc: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Tested-by: Michele Baldessari <michele@acksyn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>