ipv6: Make v4-mapped bindings consistent with IPv4

Binding to a v4-mapped address on an AF_INET6 socket should
produce the same result as binding to an IPv4 address on
AF_INET socket.  The two are interchangable as v4-mapped
address is really a portability aid.

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