selftests: mptcp: support MPTCP_PM_CMD_ANNOUNCE

This change updates the "pm_nl_ctl" testing sample with an "ann"
(announce) option to support the newly added netlink interface command
MPTCP_PM_CMD_ANNOUNCE to issue ADD_ADDR advertisements over the
chosen MPTCP connection.

E.g. ./pm_nl_ctl ann 192.168.122.75 token 823274047 id 25 dev enp1s0

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>