selftests: mptcp: join: stop transfer when check is done (part 2.2)

Use mptcp_lib_kill_wait in "userspace pm create id 0 subflow" subtest.

This new test has recently been queued to v6.6 [1] with the backport of
commit b2e2248f ("selftests: mptcp: userspace pm create id 0
subflow").

The modification here was part of commit 04b57c9e ("selftests:
mptcp: join: stop transfer when check is done (part 2)") that has been
backported to v6.6 a few months ago -- see commit 358f02b8
("selftests: mptcp: join: stop transfer when check is done (part 2)") --
but it was not modifying this subtest as it was not present in v6.6.

Link: https://git.kernel.org/pub/scm/linux/kernel/git/stable/stable-queue.git/commit/?id=bd2122541bd8

 [1]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>