KVM: TDX: Fix list_add corruption during vcpu_load()

KVM: TDX: Fix list_add corruption during vcpu_load()

During vCPU creation, a vCPU may be destroyed immediately after
kvm_arch_vcpu_create() (e.g., due to vCPU id confiliction). However, the
vcpu_load() inside kvm_arch_vcpu_create() may have associate the vCPU to
pCPU via "list_add(&tdx->cpu_list, &per_cpu(associated_tdvcpus, cpu))"
before invoking tdx_vcpu_free().

Though there's no need to invoke tdh_vp_flush() on the vCPU, failing to
dissociate the vCPU from pCPU (i.e., "list_del(&to_tdx(vcpu)->cpu_list)")
will cause list corruption of the per-pCPU list associated_tdvcpus.

Then, a later list_add() during vcpu_load() would detect list corruption
and print calltrace as shown below.

Dissociate a vCPU from its associated pCPU in tdx_vcpu_free() for the vCPUs
destroyed immediately after creation which must be in
VCPU_TD_STATE_UNINITIALIZED state.

kernel BUG at lib/list_debug.c:29!
Oops: invalid opcode: 0000 [#2] SMP NOPTI
RIP: 0010:__list_add_valid_or_report+0x82/0xd0

Call Trace:
 <TASK>
 tdx_vcpu_load+0xa8/0x120
 vt_vcpu_load+0x25/0x30
 kvm_arch_vcpu_load+0x81/0x300
 vcpu_load+0x55/0x90
 kvm_arch_vcpu_create+0x24f/0x330
 kvm_vm_ioctl_create_vcpu+0x1b1/0x53
 kvm_vm_ioctl+0xc2/0xa60
  __x64_sys_ioctl+0x9a/0xf0
 x64_sys_call+0x10ee/0x20d0
 do_syscall_64+0xc3/0x470
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Fixes: d789fa6efac9 ("KVM: TDX: Handle vCPU dissociation")
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Tested-by: Yan Zhao <yan.y.zhao@intel.com>
Tested-by: Kai Huang <kai.huang@intel.com>
Link: https://patch.msgid.link/20251030200951.3402865-29-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>