selftests/clone3: Check that the child exited cleanly

selftests/clone3: Check that the child exited cleanly

When the child exits during the clone3() selftest we use WEXITSTATUS() to
get the exit status from the process without first checking WIFEXITED() to
see if the result will be valid. This can lead to incorrect results, for
example if the child exits due to signal. Add a WIFEXTED() check and report
any non-standard exit as a failure, using EXIT_FAILURE as the exit status
for call_clone3() since we otherwise report 0 or negative errnos.

Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>