selftests: mptcp: add wrapper for showing addrs

This patch implemented a new function named pm_nl_show_endpoints(), wrapped
the PM netlink commands 'ip mptcp endpoint show' and 'pm_nl_ctl dump' in
it, used a new argument 'ip_mptcp' to choose which one to use to show all
the PM endpoints.

Used this wrapper in do_transfer() instead of using the pm_nl_ctl commands
directly.

The original 'pos+=5' in the remoing tests only works for the output of
'pm_nl_ctl show':

  id 1 flags subflow 10.0.1.1

It doesn't work for the output of 'ip mptcp endpoint show':

  10.0.1.1 id 1 subflow

So implemented a more flexible approach to get the address ID from the PM
dump output to fit for both commands.

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>