KVM: fix rcu usage warning in kvm_arch_vcpu_ioctl_set_sregs()

Fix:

[ 1001.499596] ===================================================
[ 1001.499599] [ INFO: suspicious rcu_dereference_check() usage. ]
[ 1001.499601] ---------------------------------------------------
[ 1001.499604] include/linux/kvm_host.h:301 invoked rcu_dereference_check() without protection!
	......
[ 1001.499636] Pid: 6035, comm: qemu-system-x86 Not tainted 2.6.37-rc6+ #62
[ 1001.499638] Call Trace:
[ 1001.499644]  [] lockdep_rcu_dereference+0x9d/0xa5
[ 1001.499653]  [] gfn_to_memslot+0x8d/0xc8 [kvm]
[ 1001.499661]  [] gfn_to_hva+0x16/0x3f [kvm]
[ 1001.499669]  [] kvm_read_guest_page+0x1e/0x5e [kvm]
[ 1001.499681]  [] kvm_read_guest_page_mmu+0x53/0x5e [kvm]
[ 1001.499699]  [] load_pdptrs+0x3f/0x9c [kvm]
[ 1001.499705]  [] ? vmx_set_cr0+0x507/0x517 [kvm_intel]
[ 1001.499717]  [] kvm_arch_vcpu_ioctl_set_sregs+0x1f3/0x3c0 [kvm]
[ 1001.499727]  [] kvm_vcpu_ioctl+0x6a5/0xbc5 [kvm]

Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

arch/x86/kvm/x86.c

@@ -5574,7 +5574,7 @@ int kvm_arch_vcpu_ioctl_set_sregs(struct kvm_vcpu *vcpu,
 				  struct kvm_sregs *sregs)
 {
 	int mmu_reset_needed = 0;
-	int pending_vec, max_bits;
+	int pending_vec, max_bits, idx;
 	struct desc_ptr dt;
 
 	dt.size = sregs->idt.limit;
@@ -5603,10 +5603,13 @@ int kvm_arch_vcpu_ioctl_set_sregs(struct kvm_vcpu *vcpu,
 	kvm_x86_ops->set_cr4(vcpu, sregs->cr4);
 	if (sregs->cr4 & X86_CR4_OSXSAVE)
 		update_cpuid(vcpu);
+
+	idx = srcu_read_lock(&vcpu->kvm->srcu);
 	if (!is_long_mode(vcpu) && is_pae(vcpu)) {
 		load_pdptrs(vcpu, vcpu->arch.walk_mmu, kvm_read_cr3(vcpu));
 		mmu_reset_needed = 1;
 	}
+	srcu_read_unlock(&vcpu->kvm->srcu, idx);
 
 	if (mmu_reset_needed)
 		kvm_mmu_reset_context(vcpu);