fat: Fix corrupt inode flags when remove ATTR_SYS flag

We are clearly missing '~' in fat_ioctl_set_attributes().

Cc: <stable@kernel.org>
Reported-by: Dmitry Dmitriev <dimondmm@yandex.ru>
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>