irq_work: Make irq_work_queue() NMI-safe again

Someone carelessly put NMI unsafe code in irq_work_queue(), breaking
just about every single user. Also, someone has a terrible comment
style.

Fixes: e2b5bcf9 ("irq_work: record irq_work_queue() call stack")
Reported-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/YL+uBq8LzXXZsYVf@hirez.programming.kicks-ass.net