x86/idt: Simplify the idt_setup_apic_and_irq_gates()

x86/idt: Simplify the idt_setup_apic_and_irq_gates()

The idt_setup_apic_and_irq_gates() sets the gates from
FIRST_EXTERNAL_VECTOR up to FIRST_SYSTEM_VECTOR first. then secondly, from
FIRST_SYSTEM_VECTOR to NR_VECTORS, it takes both APIC=y and APIC=n into
account.

But for APIC=n, the FIRST_SYSTEM_VECTOR is equal to NR_VECTORS, all
vectors has been set at the first step.

Simplify the second step, make it just work for APIC=y.

Signed-off-by: Dou Liyang <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]