[PATCH bpf-next v3 1/3] selftests/bpf: Add mptcp subflow example

["Matthieu Baerts (NGI0)" <matttbe@xxxxxxxxxx>]

From: Nicolas Rybowski <nicolas.rybowski@xxxxxxxxxxxx>

Move Nicolas' patch into bpf selftests directory. This example adds a
different mark (SO_MARK) on each subflow, and changes the TCP CC only on
the first subflow.