x86/kvm: Make kvm_async_pf_enabled __ro_after_init

kvm_async_pf_enabled is only ever enabled in __init kvm_guest_init(), so
mark it as __ro_after_init.

Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Link: https://lore.kernel.org/r/20240313180106.2917308-4-vschneid@redhat.com