selftests: mptcp: add pm_nl_set_endpoint helper

selftests: mptcp: add pm_nl_set_endpoint helper

This patch moves endpoint settings out of do_transfer() into a new
helper pm_nl_set_endpoint(). And invoke this helper in do_transfer().
This makes the code much more clearer.

Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20230623-send-net-next-20230623-v1-7-a883213c8ba9@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>