random: vDSO: don't use 64-bit atomics on 32-bit architectures

Performing SMP atomic operations on u64 fails on powerpc32:

    CC      drivers/char/random.o
  In file included from <command-line>:
  drivers/char/random.c: In function 'crng_reseed':
  ././include/linux/compiler_types.h:510:45: error: call to '__compiletime_assert_391' declared with attribute error: Need native word sized stores/loads for atomicity.
    510 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
        |                                             ^
  ././include/linux/compiler_types.h:491:25: note: in definition of macro '__compiletime_assert'
    491 |                         prefix ## suffix();                             \
        |                         ^~~~~~
  ././include/linux/compiler_types.h:510:9: note: in expansion of macro '_compiletime_assert'
    510 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
        |         ^~~~~~~~~~~~~~~~~~~
  ././include/linux/compiler_types.h:513:9: note: in expansion of macro 'compiletime_assert'
    513 |         compiletime_assert(__native_word(t),                            \
        |         ^~~~~~~~~~~~~~~~~~
  ./arch/powerpc/include/asm/barrier.h:74:9: note: in expansion of macro 'compiletime_assert_atomic_type'
     74 |         compiletime_assert_atomic_type(*p);                             \
        |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  ./include/asm-generic/barrier.h:172:55: note: in expansion of macro '__smp_store_release'
    172 | #define smp_store_release(p, v) do { kcsan_release(); __smp_store_release(p, v); } while (0)
        |                                                       ^~~~~~~~~~~~~~~~~~~
  drivers/char/random.c:286:9: note: in expansion of macro 'smp_store_release'
    286 |         smp_store_release(&__arch_get_k_vdso_rng_data()->generation, next_gen + 1);
        |         ^~~~~~~~~~~~~~~~~

The kernel-side generation counter in the random driver is handled as an
unsigned long, not as a u64, in base_crng and struct crng.

But on the vDSO side, it needs to be an u64, not just an unsigned long,
in order to support a 32-bit vDSO atop a 64-bit kernel.

On kernel side, however, it is an unsigned long, hence a 32-bit value on
32-bit architectures, so just cast it to unsigned long for the
smp_store_release(). A side effect is that on big endian architectures
the store will be performed in the upper 32 bits. It is not an issue on
its own because the vDSO site doesn't mind the value, as it only checks
differences. Just make sure that the vDSO side checks the full 64 bits.
For that, the local current_generation has to be u64 as well.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>