ktest: Fix child exit code processing

BugLink: http://bugs.launchpad.net/bugs/1673541

commit 32677207 upstream.

The child_exit errno needs to be shifted by 8 bits to compare against the
return values for the bisect variables.

Fixes: c5dacb88 ("ktest: Allow overriding bisect test results")
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>

tools/testing/ktest/ktest.pl

@@ -2629,7 +2629,7 @@ sub do_run_test {
     }
 
     waitpid $child_pid, 0;
-    $child_exit = $?;
+    $child_exit = $? >> 8;
 
     my $end_time = time;
     $test_time = $end_time - $start_time;