KVM: x86/mmu: Stop passing "direct" to mmu_alloc_root()

The "direct" argument is vcpu->arch.mmu->root_role.direct,
because unlike non-root page tables, it's impossible to have
a direct root in an indirect MMU.  So just use that.

Suggested-by: Lai Jiangshan <jiangshanlai@gmail.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20220516232138.1783324-4-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>