u64_stats: Disable preemption on 32bit UP+SMP PREEMPT_RT during updates.

[ Upstream commit 3c118547 ]

On PREEMPT_RT the seqcount_t for synchronisation is required on 32bit
architectures even on UP because the softirq (and the threaded IRQ handler) can
be preempted.

With the seqcount_t for synchronisation, a reader with higher priority can
preempt the writer and then spin endlessly in read_seqcount_begin() while the
writer can't make progress.

To avoid such a lock up on PREEMPT_RT the writer must disable preemption during
the update. There is no need to disable interrupts because no writer is using
this API in hard-IRQ context on PREEMPT_RT.

Disable preemption on 32bit-RT within the u64_stats write section.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: 38a15d0a ("u64_stats: fix u64_stats_init() for lockdep when used repeatedly in one file")
Signed-off-by: Sasha Levin <sashal@kernel.org>