mm, asm-generic: define PUD_SHIFT in <asm-generic/4level-fixup.h>

If an architecure uses <asm-generic/4level-fixup.h>, build fails if we
try to use PUD_SHIFT in generic code:

   In file included from arch/microblaze/include/asm/bug.h:1:0,
                    from include/linux/bug.h:4,
                    from include/linux/thread_info.h:11,
                    from include/asm-generic/preempt.h:4,
                    from arch/microblaze/include/generated/asm/preempt.h:1,
                    from include/linux/preempt.h:18,
                    from include/linux/spinlock.h:50,
                    from include/linux/mmzone.h:7,
                    from include/linux/gfp.h:5,
                    from include/linux/slab.h:14,
                    from mm/mmap.c:12:
   mm/mmap.c: In function 'exit_mmap':
>> mm/mmap.c:2858:46: error: 'PUD_SHIFT' undeclared (first use in this function)
       round_up(FIRST_USER_ADDRESS, PUD_SIZE) >> PUD_SHIFT);
                                                 ^
   include/asm...