libata: introduce notion of separate hardware tags

Rigth now these are the same, but drivers should be using ->hw_tag
for their command setup and issue.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Tejun Heo <tj@kernel.org>
diff --git a/include/linux/libata.h b/include/linux/libata.h
index 1795fec..07a8d54 100644
--- a/include/linux/libata.h
+++ b/include/linux/libata.h
@@ -637,7 +637,8 @@ struct ata_queued_cmd {
 	u8			cdb[ATAPI_CDB_LEN];
 
 	unsigned long		flags;		/* ATA_QCFLAG_xxx */
-	unsigned int		tag;
+	unsigned int		tag;		/* libata core tag */
+	unsigned int		hw_tag;		/* driver tag */
 	unsigned int		n_elem;
 	unsigned int		orig_n_elem;