UBI: introduce UBI_ALL constant

Joel will use it in his 'ubi_flush()' extention to specify all eraseblocks.
Also amend the comment for UBI_UNKNOWN - it is used beyond attaching info
structure now.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
diff --git a/include/linux/mtd/ubi.h b/include/linux/mtd/ubi.h
index 9838dce..f636f5d 100644
--- a/include/linux/mtd/ubi.h
+++ b/include/linux/mtd/ubi.h
@@ -25,6 +25,9 @@
 #include <linux/types.h>
 #include <mtd/ubi-user.h>
 
+/* All voumes/LEBs */
+#define UBI_ALL -1
+
 /*
  * enum ubi_open_mode - UBI volume open mode constants.
  *