Squashfs: fix f_pos check in get_dir_index_using_offset

One off error in the f_pos check.  If f_pos is 3 or less don't
bother reading the index because we're at the start of the
directory, and we obviously already know where that is on disk.

This eliminates an unnecessary read.
Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>

fs/squashfs/dir.c

@@ -64,7 +64,7 @@ static int get_dir_index_using_offset(struct super_block *sb,
 	 * is offset by 3 because we invent "." and ".." entries which are
 	 * not actually stored in the directory.
 	 */
-	if (f_pos < 3)
+	if (f_pos <= 3)
 		return f_pos;
 	f_pos -= 3;