KVM: x86/mmu: Snapshot shadow_phys_bits when kvm.ko is loaded

KVM: x86/mmu: Snapshot shadow_phys_bits when kvm.ko is loaded

Snapshot shadow_phys_bits when kvm.ko is loaded, not when a vendor module
is loaded, to guard against usage of shadow_phys_bits before it is
initialized.  The computation isn't vendor specific in any way, i.e. there
there is no reason to wait to snapshot the value until a vendor module is
loaded, nor is there any reason to recompute the value every time a vendor
module is loaded.

Opportunistically convert it from "read mostly" to "read-only after init".

Link: https://lore.kernel.org/r/20240423221521.2923759-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>