ktest: Fix child exit code processing

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")
Cc: stable@vger.kernel.org
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

tools/testing/ktest/ktest.pl

@@ -2633,7 +2633,7 @@ sub do_run_test {
     }
 
     waitpid $child_pid, 0;
-    $child_exit = $?;
+    $child_exit = $? >> 8;
 
     my $end_time = time;
     $test_time = $end_time - $start_time;