fat_valid_media() isn't for userspace

Commit 73f20e58

 ("FAT_VALID_MEDIA():
remove pointless test") wrongly added the new fat_valid_media() function
to the userspace-visible part of include/linux/msdos_fs.h

Move it to the part of include/linux/msdos_fs.h that is not exported to
userspace.

Reported-by: Onur Küçük <onur@pardus.org.tr>
Reported-by: S.Çağlar Onur <caglar@pardus.org.tr>
Signed-off-by: Adrian Bunk <bunk@kernel.org>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>