selftests: mptcp: update output info of chk_rm_nr

This patch updated the output info of chk_rm_nr. Renamed 'sf' to 'rmsf',
which means 'remove subflow'. Added the display of whether the inverted
namespaces has been used to check the mib counters.

The new output looks like this:

 002 remove multiple subflows          syn[ ok ] - synack[ ok ] - ack[ ok ]
                                       rm [ ok ] - rmsf  [ ok ]
 003 remove single address             syn[ ok ] - synack[ ok ] - ack[ ok ]
                                       add[ ok ] - echo  [ ok ]
                                       rm [ ok ] - rmsf  [ ok ]   invert

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