ide-cd: fix DMA alignment regression

e5318b531b008c79d2a0c0df06a7b8628da38e2f ("ide: use the dma safe check for
REQ_TYPE_ATA_PC") introduced a regression which caused some ATAPI drives to
turn off DMA for REQ_TYPE_BLOCK_PC commands while burning and thus degrading
performance and ultimately causing an excessive amount of underruns.

The issue is documented also in:
http://bugzilla.kernel.org/show_bug.cgi?id=11742.

Signed-off-by: Borislav Petkov <petkovbb@gmail.com>
Cc: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Tested-by: Valerio Passini <valerio.passini@unicam.it>
[bart: fixup patch description per comments from Sergei Shtylyov]
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>

drivers/ide/ide-cd.c

@@ -1250,7 +1250,8 @@ static void cdrom_do_block_pc(ide_drive_t *drive, struct request *rq)
 		 * separate masks.
 		 */
 		alignment = queue_dma_alignment(q) | q->dma_pad_mask;
-		if ((unsigned long)buf & alignment || rq->data_len & alignment
+		if ((unsigned long)buf & alignment
+		    || rq->data_len & q->dma_pad_mask
 		    || object_is_on_stack(buf))
 			drive->dma = 0;
 	}