selftests: mptcp: rm subflow with v4/v4mapped addr

Now both a v4 address and a v4-mapped address are supported when
destroying a userspace pm subflow, this patch adds a second subflow
to "userspace pm add & remove address" test, and two subflows could
be removed two different ways, one with the v4mapped and one with v4.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/387
Fixes: 48d73f60

 ("selftests: mptcp: update userspace pm addr tests")
Cc: stable@vger.kernel.org
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240223-upstream-net-20240223-misc-fixes-v1-2-162e87e48497@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>