selftests/bpf: Make start_mptcp_server static

start_mptcp_server() shouldn't be a public helper, it only be used in
MPTCP tests. This patch moves it into prog_tests/mptcp.c, and implenments
it using make_sockaddr() and start_server_addr() instead of using
start_server_proto().

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Link: https://lore.kernel.org/r/50ec7049e280c60a2924937940851f8fee2b73b8.1714014697.git.tanggeliang@kylinos.cn


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>