KVM: x86/mmu: Passing up the error state of mmu_alloc_shadow_roots()

Just like on the optional mmu_alloc_direct_roots() path, once shadow
path reaches "r = -EIO" somewhere, the caller needs to know the actual
state in order to enter error handling and avoid something worse.

Fixes: 4a38162e ("KVM: MMU: load PDPTRs outside mmu_lock")
Signed-off-by: Like Xu <likexu@tencent.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220301124941.48412-1-likexu@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>