selftests: mptcp: sockopt: use IPPROTO_MPTCP for getaddrinfo

getaddrinfo MPTCP is recently supported in glibc and IPPROTO_MPTCP for
getaddrinfo is used in mptcp_connect.c. But in mptcp_sockopt.c and
mptcp_inq.c, IPPROTO_TCP are still used for getaddrinfo, So this patch
updates them.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250502-net-next-mptcp-sft-inc-cover-v1-2-68eec95898fb@kernel.org


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