KVM: x86/mmu: Allow NULL @vcpu in kvm_mmu_find_shadow_page()

Allow @vcpu to be NULL in kvm_mmu_find_shadow_page() (and its only
caller __kvm_mmu_get_shadow_page()). @vcpu is only required to sync
indirect shadow pages, so it's safe to pass in NULL when looking up
direct shadow pages.

This will be used for doing eager page splitting, which allocates direct
shadow pages from the context of a VM ioctl without access to a vCPU
pointer.

Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20220516232138.1783324-14-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>