mtd: nand: revive "nand scrub" command

Since commit ff94bc40 (mtd, ubi, ubifs: resync with Linux-3.14),
the "nand scrub" command has not been working.
Signed-off-by: NMasahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Scott Wood <scottwood@freescale.com>
Cc: Heiko Schocher <hs@denx.de>

drivers/mtd/nand/nand_base.c

@@ -2905,7 +2905,7 @@ int nand_erase_nand(struct mtd_info *mtd, struct erase_info *instr,
 		WATCHDOG_RESET();
 
 		/* Check if we have a bad block, we do not erase bad blocks! */
-		if (nand_block_checkbad(mtd, ((loff_t) page) <<
+		if (!instr->scrub && nand_block_checkbad(mtd, ((loff_t) page) <<
 					chip->page_shift, 0, allowbbt)) {
 			pr_warn("%s: attempt to erase a bad block at page 0x%08x\n",
 				    __func__, page);