selftests: mptcp: ip_mptcp option for more scripts

This patch adds '-i' option for mptcp_sockopt.sh, pm_netlink.sh, and
simult_flows.sh, to use 'ip mptcp' command in the tests instead of
'pm_nl_ctl'. Update usage() correspondingly.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>