ubi,ubifs: sync with linux v4.2

sync with linux v4.2

commit 64291f7db5bd8150a74ad2036f1037e6a0428df2
Author: Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sun Aug 30 11:34:09 2015 -0700

    Linux 4.2

This update is needed, as it turned out, that fastmap
was in experimental/broken state in kernel v3.15, which
was the last base for U-Boot.

Signed-off-by: Heiko Schocher <hs@denx.de>
Tested-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
diff --git a/README b/README
index d18df54..39cdd57 100644
--- a/README
+++ b/README
@@ -3494,6 +3494,10 @@
 		without a fastmap.
 		default: 0
 
+		CONFIG_MTD_UBI_FM_DEBUG
+		Enable UBI fastmap debug
+		default: 0
+
 - UBIFS support
 		CONFIG_CMD_UBIFS