During backport f7eea636 ("KVM: nVMX: handle page fault in vmread"),
there was a mistake the exception reference should be passed to function
kvm_write_guest_virt_system, instead of NULL, other wise, we will get
NULL pointer deref, eg

kvm-unit-test triggered a NULL pointer deref below:
[  948.518437] kvm [24114]: vcpu0, guest rIP: 0x407ef9 kvm_set_msr_common: MSR_IA32_DEBUGCTLMSR 0x3, nop
[  949.106464] BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
[  949.106707] PGD 0 P4D 0
[  949.106872] Oops: 0002 [#1] SMP
[  949.107038] CPU: 2 PID: 24126 Comm: qemu-2.7 Not tainted 4.19.77-pserver #4.19.77-1+feature+daily+update+20191005.1625+a4168bb~deb9
[  949.107283] Hardware name: Dell Inc. Precision Tower 3620/09WH54, BIOS 2.7.3 01/31/2018
[  949.107549] RIP: 0010:kvm_write_guest_virt_system+0x12/0x40 [kvm]
[  949.107719] Code: c0 5d 41 5c 41 5d 41 5e 83 f8 03 41 0f 94 c0 41 c1 e0 02 e9 b0 ed ff ff 0f 1f 44 00 00 48 89 f0 c6 87 59 56 00 00 01 48 89 d6 <49> c7 00 00 00 00 00 89 ca 49 c7 40 08 00 00 00 00 49 c7 40 10 00
[  949.108044] RSP: 0018:ffffb31b0a953cb0 EFLAGS: 00010202
[  949.108216] RAX: 000000000046b4d8 RBX: ffff9e9f415b0000 RCX: 0000000000000008
[  949.108389] RDX: ffffb31b0a953cc0 RSI: ffffb31b0a953cc0 RDI: ffff9e9f415b0000
[  949.108562] RBP: 00000000d2e14928 R08: 0000000000000000 R09: 0000000000000000
[  949.108733] R10: 0000000000000000 R11: 0000000000000000 R12: ffffffffffffffc8
[  949.108907] R13: 0000000000000002 R14: ffff9e9f4f26f2e8 R15: 0000000000000000
[  949.109079] FS:  00007eff8694c700(0000) GS:ffff9e9f51a80000(0000) knlGS:0000000031415928
[  949.109318] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  949.109495] CR2: 0000000000000000 CR3: 00000003be53b002 CR4: 00000000003626e0
[  949.109671] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  949.109845] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  949.110017] Call Trace:
[  949.110186]  handle_vmread+0x22b/0x2f0 [kvm_intel]
[  949.110356]  ? vmexit_fill_RSB+0xc/0x30 [kvm_intel]
[  949.110549]  kvm_arch_vcpu_ioctl_run+0xa98/0x1b30 [kvm]
[  949.110725]  ? kvm_vcpu_ioctl+0x388/0x5d0 [kvm]
[  949.110901]  kvm_vcpu_ioctl+0x388/0x5d0 [kvm]
[  949.111072]  do_vfs_ioctl+0xa2/0x620
Signed-off-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>