u64_stats: Introduce IRQs disabled helpers

Introduce light versions of u64_stats helpers for context where
either preempt or IRQs are disabled. This way we can make this library
usable by scheduler irqtime accounting which currenty implement its
ad-hoc version.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Wanpeng Li <wanpeng.li@hotmail.com>
Link: http://lkml.kernel.org/r/1474849761-12678-4-git-send-email-fweisbec@gmail.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>