zdtm.sh: $pid has been renamed to $PID, but one was skipped

Reported-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/zdtm.sh

@@ -180,7 +180,7 @@ run_test()
 		return 1
 	}
 	if expr " $ARGS" : ' -s' > /dev/null; then
-		save_fds $pid  $ddump/dump.fd.after
+		save_fds $PID  $ddump/dump.fd.after
 		diff_fds $ddump/dump.fd $ddump/dump.fd.after || return 1
 		killall -CONT $tname
 	else