keyhandler: rework process of nonirq keyhandler

keyhandler: rework process of nonirq keyhandler

Keyhandler may run for a long time in serial port driver's
timer handler on the large machine with a lot of physical
cpus(e,g dump_timerq()) when serial port driver works in
the poll mode(via the exception mechanism).

If a timer handler runs a long time, it will block nmi_timer_fn()
to feed NMI watchdog and cause Xen hypervisor panic. Inserting
process_pending_softirqs() in timer handler will not help. when timer
interrupt arrives, timer subsystem calls all expired timer handlers
before programming next timer interrupt. There is no timer interrupt
arriving to trigger timer softirq during run a timer handler.

This patch is to fix the issue to make nonirq keyhandler run in
tasklet when receive debug key from serial port.

Signed-off-by: Lan Tianyu <tianyu.lan@intel.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>