mptcp: support MPJoin with IPv4 mapped in v6 sk

With an IPv4 mapped in v6 socket, we were trying to call inet6_bind()
with an IPv4 address resulting in a -EINVAL error because the given
addr_len -- size of the address structure -- was too short.

We now make sure to use address structures for the same family as the
MPTCP socket for both the bind() and the connect(). It means we convert
v4 addresses to v4 mapped in v6 or the opposite if needed.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/122


Co-developed-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>