KVM: MMU: fix missing locking in alloc_mmu_pages

n_requested_mmu_pages/n_free_mmu_pages are used by
kvm_mmu_change_mmu_pages to calculate the number of pages to zap.

alloc_mmu_pages, called from the vcpu initialization path, modifies this
variables without proper locking, which can result in a negative value
in kvm_mmu_change_mmu_pages (say, with cpu hotplug).
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

arch/x86/kvm/mmu.c

@@ -2728,12 +2728,14 @@ static int alloc_mmu_pages(struct kvm_vcpu *vcpu)
 
 	ASSERT(vcpu);
 
+	spin_lock(&vcpu->kvm->mmu_lock);
 	if (vcpu->kvm->arch.n_requested_mmu_pages)
 		vcpu->kvm->arch.n_free_mmu_pages =
 					vcpu->kvm->arch.n_requested_mmu_pages;
 	else
 		vcpu->kvm->arch.n_free_mmu_pages =
 					vcpu->kvm->arch.n_alloc_mmu_pages;
+	spin_unlock(&vcpu->kvm->mmu_lock);
 	/*
 	 * When emulating 32-bit mode, cr3 is only 32 bits even on x86_64.
 	 * Therefore we need to allocate shadow page tables in the first