selftests: mptcp: add set_flags command in pm_nl_ctl

This patch added the set_flags command in pm_nl_ctl, currently we can only
set two flags: backup and nobackup. The set_flags command can be used like
this:

 # pm_nl_ctl set 10.0.0.1 flags backup
 # pm_nl_ctl set 10.0.0.1 flags nobackup

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>