genirq: Switch to irq_get_nr_irqs()

genirq: Switch to irq_get_nr_irqs()

Use the irq_get_nr_irqs() function instead of the global variable
'nr_irqs'. Cache the result of this function in a local variable in
order not to rely on CSE (common subexpression elimination). Prepare
for changing 'nr_irqs' from an exported global variable into a variable
with file scope.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20241015190953.1266194-22-bvanassche@acm.org