Revert "UBI: use mtd->writebufsize to set minimal I/O unit size"

This reverts commit a121f643993474548fe98144514c50dd4f3dbe76.

Unfortunately, this commit breaks UBIFS backward compatibility and
makes new UBIFS refuse older UBIFS-formatted media:

UBIFS error: validate_sb: min. I/O unit mismatch: 8 in superblock, 64 real

Thus, we have to revert this patch and work on a better solution.

Reported-by: Holger Brunck <holger.brunck@keymile.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

drivers/mtd/ubi/build.c

@@ -672,33 +672,7 @@ static int io_init(struct ubi_device *ubi)
 		ubi->nor_flash = 1;
 	}
 
-	/*
-	 * Set UBI min. I/O size (@ubi->min_io_size). We use @mtd->writebufsize
-	 * for these purposes, not @mtd->writesize. At the moment this does not
-	 * matter for NAND, because currently @mtd->writebufsize is equivalent to
-	 * @mtd->writesize for all NANDs. However, some CFI NOR flashes may
-	 * have @mtd->writebufsize which is multiple of @mtd->writesize.
-	 *
-	 * The reason we use @mtd->writebufsize for @ubi->min_io_size is that
-	 * UBI and UBIFS recovery algorithms rely on the fact that if there was
-	 * an unclean power cut, then we can find offset of the last corrupted
-	 * node, align the offset to @ubi->min_io_size, read the rest of the
-	 * eraseblock starting from this offset, and check whether there are
-	 * only 0xFF bytes. If yes, then we are probably dealing with a
-	 * corruption caused by a power cut, if not, then this is probably some
-	 * severe corruption.
-	 *
-	 * Thus, we have to use the maximum write unit size of the flash, which
-	 * is @mtd->writebufsize, because @mtd->writesize is the minimum write
-	 * size, not the maximum.
-	 */
-	if (ubi->mtd->type == MTD_NANDFLASH)
-		ubi_assert(ubi->mtd->writebufsize == ubi->mtd->writesize);
-	else if (ubi->mtd->type == MTD_NORFLASH)
-		ubi_assert(ubi->mtd->writebufsize % ubi->mtd->writesize == 0);
-
-	ubi->min_io_size = ubi->mtd->writebufsize;
-
+	ubi->min_io_size = ubi->mtd->writesize;
 	ubi->hdrs_min_io_size = ubi->mtd->writesize >> ubi->mtd->subpage_sft;
 
 	/*