selftests: mptcp: netlink: add 'limits' helpers

The output format of 'ip mptcp limits' command is much different from
that of 'pm_nl_ctl limits' command.

This patch adds format_limits() helper to format the outputs of these
two commands to hide the difference. get_limits() has been added to show
the limits.

Use these two helpers in pm_netlink.sh to replace all 'pm_nl_ctl limits'
commands and outputs.

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>