mtd: mtdoops: Fix the size of the header read buffer.

The read buffer size depends on the MTDOOPS_HEADER_SIZE.

Tested: Changed the header size, it doesn't panic, header is still
read/written correctly.
Signed-off-by: Jean-Marc Eurin <jmeurin@google.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220421234244.2172003-2-jmeurin@google.com

drivers/mtd/mtdoops.c

@@ -223,7 +223,7 @@ static void find_next_position(struct mtdoops_context *cxt)
 {
 	struct mtd_info *mtd = cxt->mtd;
 	int ret, page, maxpos = 0;
-	u32 count[2], maxcount = 0xffffffff;
+	u32 count[MTDOOPS_HEADER_SIZE/sizeof(u32)], maxcount = 0xffffffff;
 	size_t retlen;
 
 	for (page = 0; page < cxt->oops_pages; page++) {