selftests: mptcp: interpret \n as a new line

In case of errors, this message was printed:

  (...)
  # read: Resource temporarily unavailable
  #  client exit code 0, server 3
  # \nnetns ns1-0-BJlt5D socket stat for 10003:
  (...)

Obviously, the idea was to add a new line before the socket stat and not
print "\nnetns".

Fixes: b08fbf24 ("selftests: add test-cases for MPTCP MP_JOIN")
Fixes: 048d19d4

 ("mptcp: add basic kselftest for mptcp")
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>