selftests: mptcp: use pm_nl endpoint ops

Use those newly added pm_nl endpoint ops helpers to replace all 'pm_nl_ctl'
commands with 'limits', 'add', 'del', 'flush', 'show' and 'set' arguments
in scripts mptcp_sockopt.sh and simult_flows.sh.

In pm_netlink.sh, add wrappers of there helpers to make the function names
shorter. Then use the wrappers to replace all 'pm_nl_ctl' commands.

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>