selftests: mptcp: check MP_FAIL response mibs

This patch extends chk_fail_nr to check the MP_FAIL response mibs.

Add a new argument invert for chk_fail_nr to allow it can check the
MP_FAIL TX and RX mibs from the opposite direction.

When the infinite map is received before the MP_FAIL response, the
response will be lost. A '-' can be added into fail_tx or fail_rx to
represent that MP_FAIL response TX or RX can be lost when doing the
checks.

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