genirq: Reuse irq_thread_fn() for forced thread case

rq_forced_thread_fn() uses the same action callback as the non-forced
variant but with different locking decorations.  Reuse irq_thread_fn() here
to make that clear.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20241119104339.2112455-3-andriy.shevchenko@linux.intel.com