selftests: mptcp: set ip_mptcp in command line

This patch added a command line option '-i' for mptcp_join.sh to use
'ip mptcp' commands instead of using 'pm_nl_ctl' commands to deal with
PM netlink.

Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>