staging/lustre/ptlrpc: fix ptlrpc_stop_pinger logic

It was introduced due to a patch hunk when porting
commit 20802057 (staging/lustre/ptlrpc: race in pinger).

Cc: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Peng Tao <bergwolf@gmail.com>
Cc: stable <stable@vger.kernel.org> # 3.12
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>