smp: Warn on function calls from softirq context

smp: Warn on function calls from softirq context

It's clearly documented that smp function calls cannot be invoked from
softirq handling context. Unfortunately nothing enforces that or emits a
warning.

A single function call can be invoked from softirq context only via
smp_call_function_single_async().

The only legit context is task context, so add a warning to that effect.

Reported-by: luferry <[email protected]>
Signed-off-by: Peter Zijlstra <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]