selftests: mptcp: add test for extra_subflows underflow on userspace PM

selftests: mptcp: add test for extra_subflows underflow on userspace PM

Add a test to verify that when userspace PM fails to create a subflow
(e.g. using an unreachable address), the extra_subflows counter is not
decremented below zero.

Fixes: 77e4b94a3de6 ("mptcp: update userspace pm infos")
Cc: stable@vger.kernel.org
Signed-off-by: Tao Cui <cuitao@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260602-net-mptcp-misc-fixes-7-1-rc7-v2-6-856831229976@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>