selftests: mptcp: use IPPROTO_MPTCP for getaddrinfo

mptcp_connect.c is a startup tutorial of MPTCP programming, however
there is a lack of ai_protocol(IPPROTO_MPTCP) usage. Add comment for
getaddrinfo MPTCP support.

This patch first uses IPPROTO_MPTCP to get addrinfo, and if glibc
version is too old, it falls back to using IPPROTO_TCP.

Co-developed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: zhenwei pi <pizhenwei@bytedance.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250413-net-next-mptcp-sched-mib-sft-misc-v2-8-0f83a4350150@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>