criu-ns: use os.waitstatus_to_exitcode()

os.WEXITSTATUS() returns the process exit status and it should be used
only if WIFEXITED() is true, i.e., the process terminated normally.

os.waitstatus_to_exitcode() does the same as os.WEXITSTATUS() but it
also handles the case when the process has been terminated by a signal.

Suggested-by: Andrei Vagin <avagin@gmail.com>
Signed-off-by: Radostin Stoyanov <rstoyanov@fedoraproject.org>

scripts/criu-ns

@@ -68,7 +68,7 @@ def _wait_for_process_status(criu_pid):
         try:
             (pid, status) = os.wait()
             if pid == criu_pid:
-                return os.WEXITSTATUS(status)
+                return os.waitstatus_to_exitcode(status)
         except OSError:
             return -251