Implements the infrastructure for MPTCP sockets.

MPTCP sockets open one in-kernel TCP socket per subflow. These subflow
sockets are only managed by the MPTCP socket that owns them and are not
visible from userspace. This commit allows a userspace program to open
an MPTCP socket with:

  sock = socket(AF_INET, SOCK_STREAM, IPPROTO_MPTCP);

The resulting socket is simply a wrapper around a single regular TCP
socket, without any of the MPTCP protocol implemented over the wire.
Co-developed-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NFlorian Westphal <fw@strlen.de>
Co-developed-by: NPeter Krystad <peter.krystad@linux.intel.com>
Signed-off-by: NPeter Krystad <peter.krystad@linux.intel.com>
Co-developed-by: NMatthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: NMatthieu Baerts <matthieu.baerts@tessares.net>
Co-developed-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NChristoph Paasch <cpaasch@apple.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>