mtd: nand: Properly initialize 'mtd->bitflip_threshold' prior 'scan_bbt()' is invoked

As of edbc454 [mtd: driver _read() returns max_bitflips; mtd_read() returns -EUCLEAN], 'mtd->bitflip_threshold' must be set for mtd devices having ECC, prior any 'mtd_read()' call.
Otherwise, 'mtd_read()' will falsely return -EUCLEAN.

Normally, 'mtd->bitflip_threshold' is initialized when the MTD is added.

However, this is too late for NAND MTDs, as 'scan_bbt()' is invoked prior the existing initialization of 'mtd->bitflip_threshold'.

This is a problem since 'scan_bbt()' calls 'mtd_read()', in the case of a flash-based bad block table.
It resulted in a falsely reported bitflips indication during BBT read, which lead to constant scrubbing of the flash BBT blocks.

Initialize 'mtd->bitflip_threshold' to its default value (if not already set by the driver), prior invocation of 'scan_bbt()'.

---
- The issue was introduced in v3.5-rc1, and needs to be merged prior v3.5
- Sascha, I've credited you as reporter, is that ok with you?
- Sashca, care to test the fix on your system?

index 61805e7..0a8724e 100644
@@ -3484,6 +3484,13 @@ int nand_scan_tail(struct mtd_info *mtd)
 	/* propagate ecc info to mtd_info */
 	mtd->ecclayout = chip->ecc.layout;
 	mtd->ecc_strength = chip->ecc.strength;
+	/*
+	 * Initialize bitflip_threshold to its default prior scan_bbt() call.
+	 * scan_bbt() might invoke mtd_read(), thus bitflip_threshold must be
+	 * properly set.
+	 */
+	if (!mtd->bitflip_threshold)
+		mtd->bitflip_threshold = mtd->ecc_strength;
 
 	/* Check, if we should skip the bad block table scan */
 	if (chip->options & NAND_SKIP_BBTSCAN)

1.7.5.4