selftests: mptcp: userspace pm remove initial subflow

This patch adds a selftest for userspace PM to remove the initial
subflow.

Use userspace_pm_add_sf() to add a subflow, and pass initial IP address
to userspace_pm_rm_sf() to remove the initial subflow.

Reviewed-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20231128-send-net-next-2023107-v4-7-8d6b94150f6b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>