selftests: mptcp: userspace_pm: reduce dup code around printf

In this selftest, "printf" is always used with "stdbuf".

With a new helper, it is possible to call "stdbuf" only from one place.
This makes the code a bit clearer to read.

Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>