selftests: mptcp: use minus values for removing address numbers

This patch changes the removing addresses numbers to minus values, left
the plus values for the adding addresses numbers.

Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>