workqueue: trigger a single-CPU backtrace for stalled pools

When a CPU pool is stalled with no running worker, the task occupying the
CPU may not be a workqueue worker at all.  Trigger a single-CPU backtrace
for the stalled CPU to capture what it is currently executing.

The CPU is snapshotted under pool->lock and the backtrace is triggered
after releasing the lock to avoid any potential issues with NMI delivery.

Skip the backtrace when the CPU is offline.  A pool disassociated by CPU
hotplug keeps its pool->cpu, and an NMI to an offline CPU is never acked,
so nmi_trigger_cpumask_backtrace() would busy-wait for its full timeout
in the watchdog's timer context.

Suggested-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Tejun Heo <tj@kernel.org>

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