ANDROID: Incremental fs: fix u64 integer cast to pointer

Compiler (arm-linux-gnueabihf-gcc 8.3.0) complains about an invalid cast
of an __aligned_u64 integer to a pointer on 32-bit architectures. Using
u64_to_user_ptr() for the cast fixes the following warning:

fs/incfs/pseudo_files.c: In function ‘ioctl_create_file’:
fs/incfs/pseudo_files.c:656:10: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
  656 |          (u8 __user *)args.signature_info,
      |          ^

Bug: 183339614
Fixes: bc6a70e849cc (ANDROID: Incremental fs: Remove signature checks from kernel)
Reported-by: kernelci.org bot <bot@kernelci.org>
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Change-Id: I98a987fb83c160740796c0b4b3fdd7551880e12a