ndisc: Move ndisc_opt_addr_space() to include/net/ndisc.h.

This also makes ndisc_opt_addr_data() and ndisc_fill_addr_option()
use ndisc_opt_addr_space().

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>