selftests: mptcp: add userspace pm subflow flag

This patch adds the address flag MPTCP_PM_ADDR_FLAG_SUBFLOW in csf() in
pm_nl_ctl.c when subflow is created by a userspace PM.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>