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: Takumi Asaki <takumi.asaki@nokia.com>
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>