KVM: TDX: Use guard() to acquire kvm->lock in tdx_vm_ioctl()

KVM: TDX: Use guard() to acquire kvm->lock in tdx_vm_ioctl()

Use guard() in tdx_vm_ioctl() to tidy up the code a small amount, but more
importantly to minimize the diff of a future change, which will use
guard-like semantics to acquire and release multiple locks.

No functional change intended.

Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.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/20251030200951.3402865-25-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>