ipv6: Allow inet6_dump_addr() to handle more than 64 addresses

Apparently, inet6_dump_addr() is not able to handle more than
64 ipv6 addresses per device. We must break from inner loops
in case skb is full, or else cursor is put at the end of list.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>