ipv6: addrconf: move ipv6_chk_same_addr() to avoid forward declaration

ipv6_chk_same_addr() is only used by ipv6_add_addr_hash(),
so moving it avoids a forward declaration.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>