scsi: libata: Provide an ata_scsi_dma_need_drain stub for !CONFIG_ATA

SAS drivers can be compiled with ata support disabled.  Provide a stub so
that the drivers don't have to ifdef around wiring up
ata_scsi_dma_need_drain.

Link: https://lore.kernel.org/r/20200615064624.37317-2-hch@lst.deSigned-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

include/linux/libata.h

@@ -1092,7 +1092,11 @@ 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)
 bool ata_scsi_dma_need_drain(struct request *rq);
+#else
+#define ata_scsi_dma_need_drain NULL
+#endif
 extern int ata_sas_scsi_ioctl(struct ata_port *ap, struct scsi_device *dev,
 			    unsigned int cmd, void __user *arg);
 extern bool ata_link_online(struct ata_link *link);