staging/lustre: Convert ptlrpc_at_check_timed to void

The only caller was not looking at the return value,
and liblustre, that cared about it is not part of the kernel client.

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>