ext4: fix dx_map_entry to support 256k directory blocks

The dx_map_entry structure doesn't support over 64KB block size by
current usage of its member("offs"). Because "offs" treats an offset
of copies of the ext4_dir_entry_2 structure as is. This member size is
16 bits. But real offset for over 64KB(256KB) block size needs 18
bits. However, real offset keeps 4 byte boundary, so lower 2 bits is
not used.

Therefore, we do the following to fix this limitation:
For "store": 
	we divide the real offset by 4 and then store this result to "offs" 
	member.
For "use":
	we multiply "offs" member by 4 and then use this result 
	as real offset.
Signed-off-by: Toshiyuki Okajima <toshi.okajima@jp.fujitsu.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/namei.c

@@ -749,7 +749,7 @@ static int dx_make_map(struct ext4_dir_entry_2 *de, unsigned blocksize,
 			ext4fs_dirhash(de->name, de->name_len, &h);
 			map_tail--;
 			map_tail->hash = h.hash;
-			map_tail->offs = (u16) ((char *) de - base);
+			map_tail->offs = ((char *) de - base)>>2;
 			map_tail->size = le16_to_cpu(de->rec_len);
 			count++;
 			cond_resched();
@@ -1147,7 +1147,8 @@ dx_move_dirents(char *from, char *to, struct dx_map_entry *map, int count,
 	unsigned rec_len = 0;
 
 	while (count--) {
-		struct ext4_dir_entry_2 *de = (struct ext4_dir_entry_2 *) (from + map->offs);
+		struct ext4_dir_entry_2 *de = (struct ext4_dir_entry_2 *) 
+						(from + (map->offs<<2));
 		rec_len = EXT4_DIR_REC_LEN(de->name_len);
 		memcpy (to, de, rec_len);
 		((struct ext4_dir_entry_2 *) to)->rec_len =