ipv6: check for IPv4 mapped addresses when connecting IPv6 sockets

When support for binding to 'mapped INADDR_ANY (::ffff.0.0.0.0)' was added
in 0f8d3c7a the rest of the code
wasn't told so now it's possible to bind IPv6 datagram socket to
::ffff.0.0.0.0, connect it to another IPv4 address and it will all
work except for getsockhame() which does not return the local address
as expected.

To give getsockname() something to work with check for 'mapped INADDR_ANY'
when connecting and update the in-core source addresses appropriately.

Signed-off-by: Max Matveev <makc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>