tasklets: Provide tasklet_disable_in_atomic()

Replacing the spin wait loops in tasklet_unlock_wait() with
wait_var_event() is not possible as a handful of tasklet_disable()
invocations are happening in atomic context. All other invocations are in
teardown paths which can sleep.

Provide tasklet_disable_in_atomic() and tasklet_unlock_spin_wait() to
convert the few atomic use cases over, which allows to change
tasklet_disable() and tasklet_unlock_wait() in a later step.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210309084241.563164193@linutronix.de