preempt: Provide preempt_[dis|en]able_nested()

On PREEMPT_RT enabled kernels, spinlocks and rwlocks are neither disabling
preemption nor interrupts. Though there are a few places which depend on
the implicit preemption/interrupt disable of those locks, e.g. seqcount
write sections, per CPU statistics updates etc.

To avoid sprinkling CONFIG_PREEMPT_RT conditionals all over the place, add
preempt_disable_nested() and preempt_enable_nested() which should be
descriptive enough.

Add a lockdep assertion for the !PREEMPT_RT case to catch callers which
do not have preemption disabled.

Suggested-by: Linus Torvalds <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Acked-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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