fat: fix FAT sector offsets overflow on large FAT partitions

This patch fixes several issues where sector offsets can overflow due
to being limited to 16-bits. The cases where an overflow can happen
when accessing large FAT32 partitions are:

 - length of FAT in sectors
 - start sector of root directory
 - the sector of the first cluster

These issues were observed when reading files from a 64GB FAT32
filesystem.

Signed-off-by: Aaron Williams <aaron.williams@caviumnetworks.com>
Tested-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Anatolij Gustschin <agust@denx.de>

include/fat.h

@@ -178,12 +178,12 @@ typedef struct dir_slot {
 typedef struct {
 	__u8	*fatbuf;	/* Current FAT buffer */
 	int	fatsize;	/* Size of FAT in bits */
-	__u16	fatlength;	/* Length of FAT in sectors */
+	__u32	fatlength;	/* Length of FAT in sectors */
 	__u16	fat_sect;	/* Starting sector of the FAT */
-	__u16	rootdir_sect;	/* Start sector of root directory */
+	__u32	rootdir_sect;	/* Start sector of root directory */
 	__u16	sect_size;	/* Size of sectors in bytes */
 	__u16	clust_size;	/* Size of clusters in sectors */
-	short	data_begin;	/* The sector of the first cluster, can be negative */
+	int	data_begin;	/* The sector of the first cluster, can be negative */
 	int	fatbufnum;	/* Used by get_fatent, init to -1 */
 } fsdata;