block: don't hardcode blk_qc_t -> tag mask

Use the shift/mask we use elsewhere.
Signed-off-by: Jens Axboe <axboe@fb.com>

include/linux/blk_types.h

@@ -265,7 +265,7 @@ static inline unsigned int blk_qc_t_to_queue_num(blk_qc_t cookie)
 
 static inline unsigned int blk_qc_t_to_tag(blk_qc_t cookie)
 {
-	return cookie & 0xffff;
+	return cookie & ((1u << BLK_QC_T_SHIFT) - 1);
 }
 
 #endif /* __LINUX_BLK_TYPES_H */