Provide a local lock based serialization for soft interrupts on RT which
allows the local_bh_disabled() sections and servicing soft interrupts to be
preemptible.

Provide the necessary inline helpers which allow to reuse the bulk of the
softirq processing code.
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Tested-by: NSebastian Andrzej Siewior <bigeasy@linutronix.de>
Tested-by: NPaul E. McKenney <paulmck@kernel.org>
Reviewed-by: NFrederic Weisbecker <frederic@kernel.org>
Acked-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210309085727.426370483@linutronix.de