scsi: libata: Fix the ata_scsi_dma_need_drain stub

We not only need the stub when libata is disabled, but also if it is
modular and there are built-in SAS drivers (which can happen when
SCSI_SAS_ATA is disabled).

Link: https://lore.kernel.org/r/20200620071302.462974-2-hch@lst.de
Fixes: b8f1d1e ("scsi: Wire up ata_scsi_dma_need_drain for SAS HBA drivers")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

include/linux/libata.h

@@ -1092,7 +1092,7 @@ extern int ata_scsi_ioctl(struct scsi_device *dev, unsigned int cmd,
 #define ATA_SCSI_COMPAT_IOCTL /* empty */
 #endif
 extern int ata_scsi_queuecmd(struct Scsi_Host *h, struct scsi_cmnd *cmd);
-#if IS_ENABLED(CONFIG_ATA)
+#if IS_REACHABLE(CONFIG_ATA)
 bool ata_scsi_dma_need_drain(struct request *rq);
 #else
 #define ata_scsi_dma_need_drain NULL