fat: Fix statfs->f_namelen

pathconf(, _PC_NAME_MAX) is too small for long Unicode filename on fat.

255 as max filename size on fat is Unicode UTF-16 characters.
it's not byte size.

https://bugzilla.kernel.org/show_bug.cgi?id=16469

To fix it, this returns "len * NLS_MAX_CHARSET_SIZE" instead.
Reported-by: NTakumi Asaki <takumi.asaki@nokia.com>
Signed-off-by: NOGAWA Hirofumi <hirofumi@mail.parknet.co.jp>

fs/fat/inode.c

@@ -581,7 +581,8 @@ static int fat_statfs(struct dentry *dentry, struct kstatfs *buf)
 	buf->f_bavail = sbi->free_clusters;
 	buf->f_fsid.val[0] = (u32)id;
 	buf->f_fsid.val[1] = (u32)(id >> 32);
-	buf->f_namelen = sbi->options.isvfat ? FAT_LFN_LEN : 12;
+	buf->f_namelen =
+		(sbi->options.isvfat ? FAT_LFN_LEN : 12) * NLS_MAX_CHARSET_SIZE;
 
 	return 0;
 }