KVM: x86: Drop MAX_NR_RESERVED_IOAPIC_PINS, use KVM_MAX_IRQ_ROUTES directly

KVM: x86: Drop MAX_NR_RESERVED_IOAPIC_PINS, use KVM_MAX_IRQ_ROUTES directly

Directly use KVM_MAX_IRQ_ROUTES when checking the number of routes being
defined by userspace when creating a split IRQCHIP.  The restriction has
nothing to do with the I/O APIC, e.g. most modern userspace usage is for
routing MSIs.  Breaking the unnecessary dependency on the I/O APIC will
allow burying all of ioapic.h behind CONFIG_KVM_IOAPIC=y.

No functional change intended.

Link: https://patch.msgid.link/20251206004311.479939-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>