selftests: mptcp: add invert argument for chk_rm_nr

Some of the removing testcases used two zeros as arguments for chk_rm_nr
like this: chk_rm_nr 0 0. This doesn't mean that no RM_ADDR has been sent.
It only means that RM_ADDR had been sent in the opposite direction that
chk_rm_nr is checking.

This patch added a new argument invert for chk_rm_nr to allow it can
check the RM_ADDR from the opposite direction.

Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>