[IPV6] ADDRCONF: Rename ipv6_saddr_label() to ipv6_addr_label().

This patch renames ipv6_saddr_label() to ipv6_addr_label() because
address label is used for both of source address and destination
address.

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