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

commit 452690be upstream.

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: 01cacb00 ("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>
[ Conflicts in pm_netlink.sh, because some refactoring have been done
  later on: commit 3188309c ("selftests: mptcp: netlink:
  add 'limits' helpers") and commit c99d57d0 ("selftests: mptcp: use
  pm_nl endpoint ops") are not in this version. The same operation can
  still be done at the same place, without using the new helper. ]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>