selftests: mptcp: diag: drop nlh parameter of recv_nlmsg

selftests: mptcp: diag: drop nlh parameter of recv_nlmsg

It's strange that 'nlh' variable is set to NULL in get_mptcpinfo() and then
this NULL pointer is passed to recv_nlmsg(). In fact, this variable should
be defined in recv_nlmsg(), not get_mptcpinfo().

So this patch drops this useless 'nlh' parameter of recv_nlmsg() and define
'nlh' variable in recv_nlmsg().

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/20250413-net-next-mptcp-sched-mib-sft-misc-v2-7-0f83a4350150@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>