mptcp: add basic kselftest for mptcp

Add mptcp_connect tool:
xmit two files back and forth between two processes, several net
namespaces including some adding delays, losses and reordering.
Wrapper script tests that data was transmitted without corruption.

The "-c" command line option for mptcp_connect.sh is there for debugging:

The script will use tcpdump to create one .pcap file per test case, named
according to the namespaces, protocols, and connect address in use.
For example, the first test case writes the capture to
ns1-ns1-MPTCP-MPTCP-10.0.1.1.pcap.

The stderr output from tcpdump is printed after the test completes to
show tcpdump's "packets dropped by kernel" information.

Also check that userspace can't create MPTCP sockets when mptcp.enabled
sysctl is off.

The "-b" option allows to tune/lower send buffer size.
"-m mmap" can be used to test blocking io.  Default is non-blocking
io using read/write/poll.

Will run automatically on "make kselftest".

Note that the default timeout of 45 seconds is used even if there is a
"settings" changing it to 450. 45 seconds should be enough in most cases
but this depends on the machine running the tests.

A fix to correctly read the "settings" file has been proposed upstream
but not applied yet. It is not blocking the execution of these new tests
but it would be nice to have it:

  https://patchwork.kernel.org/patch/11204935/



Co-developed-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Co-developed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Co-developed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Co-developed-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Christoph Paasch <cpaasch@apple.com>
Signed-off-by: David S. Miller <davem@davemloft.net>