FAT: buffer overflow with FAT12/16

Last commit 3831530dcb was intended
"explicitly specify FAT12/16 root directory parsing buffer size, instead
of relying on cluster size". Howver, the underlying function requires
the size of the buffer in blocks, not in bytes, and instead of passing
a double sector size a request for 1024 blocks is sent. This generates
a buffer overflow with overwriting of other structure (in the case seen,
USB structures were overwritten).

Signed-off-by: Stefano Babic <sbabic@denx.de>
CC: Mikhail Zolotaryov <lebon@lebon.org.ua>

fs/fat/fat.c

@@ -858,7 +858,7 @@ do_fat_read (const char *filename, void *buffer, unsigned long maxsize,
 		if (disk_read(cursect,
 				(mydata->fatsize == 32) ?
 				(mydata->clust_size) :
-				LINEAR_PREFETCH_SIZE,
+				LINEAR_PREFETCH_SIZE / SECTOR_SIZE,
 				do_fat_read_block) < 0) {
 			debug("Error: reading rootdir block\n");
 			return -1;