genirq: Handle missing work_struct in irq_set_affinity_notifier()

[ Upstream commit bbc4d2a7d6ff54ba923640d9a42c7bef7185fe98 ]

The backported stable commit
   59c39840 ("genirq: Prevent use-after-free and work list corruption")

added cancel_work_sync() on a work_struct element which is not available
in RT.

Replace cancel_work_sync() with kthread_cancel_work_sync() on RT.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>