u64_stats: Streamline the implementation

The u64 stats code handles 3 different cases:

  - 32bit UP
  - 32bit SMP
  - 64bit

with an unreadable #ifdef maze, which was recently expanded with PREEMPT_RT
conditionals.

Reduce it to two cases (32bit and 64bit) and drop the optimization for
32bit UP as suggested by Linus.

Use the new preempt_disable/enable_nested() helpers to get rid of the
CONFIG_PREEMPT_RT conditionals.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220825164131.402717-9-bigeasy@linutronix.de