block: add REQ_SECURE to REQ_COMMON_MASK

Add REQ_SECURE flag to REQ_COMMON_MASK so that
init_request_from_bio() can pass it to @req->cmd_flags.

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: stable@kernel.org # 2.6.36 and newer
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>