selftests: mptcp: support MPTCP_PM_CMD_REMOVE

This change updates the "pm_nl_ctl" testing sample with a "rem"
(remove) option to support the newly added netlink interface command
MPTCP_PM_CMD_REMOVE to issue a REMOVE_ADDR signal over the
chosen MPTCP connection.

E.g. ./pm_nl_ctl rem token 823274047 id 23

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>