ipv6: Disallow binding to v4-mapped address on v6-only socket.

A socket marked v6-only, can not receive or send traffic to v4-mapped
addresses.  Thus allowing binding to v4-mapped address on such a
socket makes no sense.

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>