selftests: mptcp: diag: return KSFT_FAIL not test_cnt

The test counter 'test_cnt' should not be returned in diag.sh, e.g. what
if only the 4th test fail? Will do 'exit 4' which is 'exit ${KSFT_SKIP}',
the whole test will be marked as skipped instead of 'failed'!

So we should do ret=${KSFT_FAIL} instead.

Fixes: df62f2ec ("selftests/mptcp: add diag interface tests")
Cc: stable@vger.kernel.org
Fixes: 42fb6cdd

 ("selftests: mptcp: more stable diag tests")
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>