selftests: mptcp: dump userspace addrs list

This patch adds a new helper userspace_pm_dump() to dump addresses
for the userspace PM. Use this helper to check whether an ID 0 subflow
is listed in the output of dump command after creating an ID 0 subflow
in "userspace pm create id 0 subflow" test. Dump userspace PM addresses
list in "userspace pm add & remove address" test and in "userspace pm
create destroy subflow" test.

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>