KVM: x86: fix missed SRCU usage in kvm_lapic_set_vapic_addr

Reported by syzkaller:

    [ INFO: suspicious RCU usage. ]
    4.9.0-rc4+ #47 Not tainted
    -------------------------------
    ./include/linux/kvm_host.h:536 suspicious rcu_dereference_check() usage!

    stack backtrace:
    CPU: 1 PID: 6679 Comm: syz-executor Not tainted 4.9.0-rc4+ #47
    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
     ffff880039e2f6d0 ffffffff81c2e46b ffff88003e3a5b40 0000000000000000
     0000000000000001 ffffffff83215600 ffff880039e2f700 ffffffff81334ea9
     ffffc9000730b000 0000000000000004 ffff88003c4f8420 ffff88003d3f8000
    Call Trace:
     [<     inline     >] __dump_stack lib/dump_stack.c:15
     [<ffffffff81c2e46b>] dump_stack+0xb3/0x118 lib/dump_stack.c:51
     [<ffffffff81334ea9>] lockdep_rcu_suspicious+0x139/0x180 kernel/locking/lockdep.c:4445
     [<     inline     >] __kvm_memslots include/linux/kvm_host.h:534
     [<     inline     >] kvm_memslots include/linux/kvm_host.h:541
     [<ffffffff8105d6ae>] kvm_gfn_to_hva_cache_init+0xa1e/0xce0 virt/kvm/kvm_main.c:1941
     [<ffffffff8112685d>] kvm_lapic_set_vapic_addr+0xed/0x140 arch/x86/kvm/lapic.c:2217

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Fixes: fda4e2e85589191b123d31cdc21fd33ee70f50fd
Cc: Andrew Honig <ahonig@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

arch/x86/kvm/x86.c

@@ -3431,6 +3431,7 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
 	};
 	case KVM_SET_VAPIC_ADDR: {
 		struct kvm_vapic_addr va;
+		int idx;
 
 		r = -EINVAL;
 		if (!lapic_in_kernel(vcpu))
@@ -3438,7 +3439,9 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
 		r = -EFAULT;
 		if (copy_from_user(&va, argp, sizeof va))
 			goto out;
+		idx = srcu_read_lock(&vcpu->kvm->srcu);
 		r = kvm_lapic_set_vapic_addr(vcpu, va.vapic_addr);
+		srcu_read_unlock(&vcpu->kvm->srcu, idx);
 		break;
 	}
 	case KVM_X86_SETUP_MCE: {