selftests: mptcp: sockopt: return error if wrong mark

When an error was detected when checking the marks, a message was
correctly printed mentioning the error but followed by another one
saying everything was OK and the selftest was not marked as failed as
expected.

Now the 'ret' variable is directly set to 1 in order to make sure the
exit is done with an error, similar to what is done in other functions.
While at it, the error is correctly propagated to the caller.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368
Fixes: dc65fe82

 ("selftests: mptcp: add packet mark test case")
Cc: stable@vger.kernel.org
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>