KVM: x86: return 0 in case kvm_mmu_memory_cache has min number of objects

The code tries to pre-allocate *min* number of objects, so it is ok to
return 0 when the kvm_mmu_memory_cache meets the requirement.

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>