selftests: mptcp: update userspace pm subflow tests

selftests: mptcp: update userspace pm subflow tests

To align with what is done by the in-kernel PM, update userspace pm
subflow selftests, by sending the a remove_addrs command together
before the remove_subflows command. This will get a RM_ADDR in
chk_rm_nr().

Fixes: d9a4594edabf ("mptcp: netlink: Add MPTCP_PM_CMD_REMOVE")
Fixes: 5e986ec46874 ("selftests: mptcp: userspace pm subflow tests")
Link: https://github.com/multipath-tcp/mptcp_net-next/issues/379
Cc: stable@vger.kernel.org
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>