[SCTP] ipv6: inconsistent lock state ipv6_add_addr/sctp_v6_copy_addrlist

lockdep found that dev->lock taken from softirq in ipv6_add_addr
is also taken in sctp_v6_copy_addrlist with softirqs enabled, so
lockup is possible.

Noticed-by: Simon Arlott <simon@arlott.org>
Signed-off-by: Jarek Poplawski <jarkao2@o2.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>