rcu-tasks: Avoid raw-spinlocked wakeups from call_rcu_tasks_generic()

If the caller of of call_rcu_tasks(), call_rcu_tasks_rude(),
or call_rcu_tasks_trace() holds a raw spinlock, and then if
call_rcu_tasks_generic() determines that the grace-period kthread must
be awakened, then the wakeup might acquire a normal spinlock while a
raw spinlock is held.  This results in lockdep splats when the
kernel is built with CONFIG_PROVE_RAW_LOCK_NESTING=y.

This commit therefore defers the wakeup using irq_work_queue().

It would be nice to directly invoke wakeup when a raw spinlock is not
held, but there is currently no way to check for this in all kernels.

Reported-by: Martin Lau <kafai@fb.com>
Cc: Neeraj Upadhyay <neeraj.iitr10@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>