mtd: nand: fix bug writing 1 byte less than page size

nand_do_write_ops() determines if it is writing a partial page with the
formula:
	part_pagewr = (column || writelen < (mtd->writesize - 1))

When 'writelen' is exactly 1 byte less than the NAND page size the formula
equates to zero, so the code doesn't process it as a partial write,
although it should.
As a consequence the function remains in the while(1) loop with 'writelen'
becoming 0xffffffff and iterating endlessly.

The bug may not be easy to reproduce in Linux since user space tools
usually force the padding or round-up the write size to a page-size
multiple.
This was discovered in U-Boot where the issue can be reproduced by
writing any size that is 1 byte less than a page-size multiple.
For example, on a NAND with 2K page (0x800):
	=> nand erase.part <partition>
	=> nand write $loadaddr <partition> 7ff

[Editor's note: the bug was added in commit 29072b9607, but moved
around in commit 66507c7bc8 ("mtd: nand: Add support to use nand_base
poi databuf as bounce buffer")]

Fixes: 29072b9607 ("[MTD] NAND: add subpage write support")
Signed-off-by: Hector Palacios <hector.palacios@digi.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

drivers/mtd/nand/nand_base.c

@@ -2610,7 +2610,7 @@ static int nand_do_write_ops(struct mtd_info *mtd, loff_t to,
 		int cached = writelen > bytes && page != blockmask;
 		uint8_t *wbuf = buf;
 		int use_bufpoi;
-		int part_pagewr = (column || writelen < (mtd->writesize - 1));
+		int part_pagewr = (column || writelen < mtd->writesize);
 
 		if (part_pagewr)
 			use_bufpoi = 1;