selftests: mptcp: userspace PM support for MP_PRIO signals

This change updates the testing sample (pm_nl_ctl) to exercise
the updated MPTCP_PM_CMD_SET_FLAGS command for userspace PMs to
issue MP_PRIO signals over the selected subflow.

E.g. ./pm_nl_ctl set 10.0.1.2 port 47234 flags backup token 823274047 rip 10.0.1.1 rport 50003

userspace_pm.sh has a new selftest that invokes this command.

Fixes: 259a834f

 ("selftests: mptcp: functional tests for the userspace PM type")
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Kishen Maloor <kishen.maloor@intel.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>