libata: don't use on-stack sense buffer

sense_buffer is used as DMA target and shouldn't be allocated on
stack.  Use ap->sector_buf instead.  This problem is spotted by Chuck
Ebbert.
Signed-off-by: NTejun Heo <tj@kernel.org>
Reported-by: NChuck Ebbert <cebbert@redhat.com>
Signed-off-by: NJeff Garzik <jgarzik@redhat.com>

drivers/ata/libata-eh.c

@@ -2901,7 +2901,7 @@ static int atapi_eh_clear_ua(struct ata_device *dev)
 	int i;
 
 	for (i = 0; i < ATA_EH_UA_TRIES; i++) {
-		u8 sense_buffer[SCSI_SENSE_BUFFERSIZE];
+		u8 *sense_buffer = dev->link->ap->sector_buf;
 		u8 sense_key = 0;
 		unsigned int err_mask;