selftests: mptcp: support MPTCP_PM_CMD_SUBFLOW_DESTROY

This change updates the "pm_nl_ctl" testing sample with a "dsf"
(destroy subflow) option to support the newly added netlink interface
command MPTCP_PM_CMD_SUBFLOW_DESTROY over the chosen MPTCP connection.

E.g. ./pm_nl_ctl dsf lip 10.0.2.1 lport 44567 rip 10.0.2.2 rport 56789
token 823274047

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>