selftests/userfaultfd: fix compiler warnings on 32-bit

On 32-bit:

    userfaultfd.c: In function 'locking_thread':
    userfaultfd.c:152: warning: left shift count >= width of type
    userfaultfd.c: In function 'uffd_poll_thread':
    userfaultfd.c:295: warning: cast to pointer from integer of different size
    userfaultfd.c: In function 'uffd_read_thread':
    userfaultfd.c:332: warning: cast to pointer from integer of different size

Fix the shift warning by splitting the shift in two parts, and the
integer/pointer warnigns by adding intermediate casts to "unsigned long".

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>