genirq: Introduce irq_get_nr_irqs() and irq_set_nr_irqs()

genirq: Introduce irq_get_nr_irqs() and irq_set_nr_irqs()

Prepare for changing 'nr_irqs' from an exported global variable into a
variable with file scope.

This will prevent accidental changes of assignments to a local variable
'nr_irqs' into assignments to the global 'nr_irqs' variable.

Suppose that a patch would be submitted for review that removes a
declaration of a local variable with the name 'nr_irqs' and that that patch
does not remove all assignments to that local variable. Such a patch
converts an assignment to a local variable into an assignment into a global
variable. If the 'nr_irqs' assignment is more than three lines away from
other changes, the assignment won't be included in the diff context lines
and hence won't be visible without inspecting the modified file.

With these abstraction series applied, such accidental conversions from
assignments to a local variable into an assignment to a global variable are
converted into a compilation error.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20241015190953.1266194-2-bvanassche@acm.org