fat: fix VFAT_IOCTL_READDIR_xxx and cleanup for userland

"struct dirent" is a kernel type here, but is a **different type** in
userspace!  This means both the structure and the IOCTL number is wrong!

So, this adds new "struct __fat_dirent" to generate correct IOCTL number.
And kernel stuff moves to under __KERNEL__.

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>