bcache: move from global variable to cmd struct

lib/commands/toolcontext.h

@@ -236,6 +236,17 @@ struct cmd_context {
 	const char *devicesfile;                /* from --devicesfile option */
 	struct dm_list deviceslist;             /* from --devices option, struct dm_str_list */
 
+	/*
+	 * Cache of blocks read from devices, generally blocks containing
+	 * headers and metadata from PVs.  (It functions more as an i/o layer
+	 * rather than a cache, the caching occurs when blocks are reused
+	 * between label_scan and vg_read.)
+	 * (FIXME: in some cases blocks from LVs are contained here, e.g. for
+	 * zeroing, but that's not the intended use of bcache, and those cases
+	 * should be changed to perform i/o to LVs separately.)
+	 */
+	struct bcache *dev_blocks;
+
 	/*
 	 * LV dev index.
 	 */