KVM: x86: Use ERR_PTR_USR() to return -EFAULT as a __user pointer

Use ERR_PTR_USR() when returning -EFAULT from kvm_get_attr_addr(), sparse
complains about implicitly casting the kernel pointer from ERR_PTR() into
a __user pointer.

>> arch/x86/kvm/x86.c:4342:31: sparse: sparse: incorrect type in return expression
   (different address spaces) @@     expected void [noderef] __user * @@     got void * @@
   arch/x86/kvm/x86.c:4342:31: sparse:     expected void [noderef] __user *
   arch/x86/kvm/x86.c:4342:31: sparse:     got void *
>> arch/x86/kvm/x86.c:4342:31: sparse: sparse: incorrect type in return expression
   (different address spaces) @@     expected void [noderef] __user * @@     got void * @@
   arch/x86/kvm/x86.c:4342:31: sparse:     expected void [noderef] __user *
   arch/x86/kvm/x86.c:4342:31: sparse:     got void *

No functional change intended.

Fixes: 56f289a8 ("KVM: x86: Add a helper to retrieve userspace address from kvm_device_attr")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220202005157.2545816-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>