selftests: mptcp: update userspace pm test helpers

This patch adds a new argument namespace to userspace_pm_add_addr() and
userspace_pm_add_sf() to make these two helper more versatile.

Add two more versatile helpers for userspace pm remove subflow or address:
userspace_pm_rm_addr() and userspace_pm_rm_sf(). The original test helpers
userspace_pm_rm_sf_addr_ns1() and userspace_pm_rm_sf_addr_ns2() can be
replaced by these new helpers.

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-v4-4-8d6b94150f6b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>