Fix corner case in bad block table handling.

In the unlikely event that both blocks 10 and 11 are marked as bad (on a
32 bit machine), then the process of marking block 10 as bad stomps on
cached entry for block 11.  There are (of course) other examples.

Signed-off-by: Patrick Doyle <pdoyle@irobot.com>
Reviewed-by: Richard Weinberger <richard@nod.at>
diff --git a/drivers/mtd/nand/bbt.c b/drivers/mtd/nand/bbt.c
index 84d60b8..294daee 100644
--- a/drivers/mtd/nand/bbt.c
+++ b/drivers/mtd/nand/bbt.c
@@ -127,7 +127,7 @@
 		unsigned int rbits = bits_per_block + offs - BITS_PER_LONG;
 
 		pos[1] &= ~GENMASK(rbits - 1, 0);
-		pos[1] |= val >> rbits;
+		pos[1] |= val >> (bits_per_block - rbits);
 	}
 
 	return 0;