selftests: mptcp: pm: check flush doesn't reset limits

selftests: mptcp: pm: check flush doesn't reset limits

This modification is linked to the parent commit where the received
ADD_ADDR limit was accidentally reset when the endpoints were flushed.

To validate that, the test is now flushing endpoints after having set
new limits, and before checking them.

The 'Fixes' tag here below is the same as the one from the previous
commit: this patch here is not fixing anything wrong in the selftests,
but it validates the previous fix for an issue introduced by this commit
ID.

Fixes: 01cacb00b35c ("mptcp: add netlink-based PM")
Cc: stable@vger.kernel.org
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250815-net-mptcp-misc-fixes-6-17-rc2-v1-3-521fe9957892@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>