KVM: don't use anon_inode_getfd() before possible failures

Once anon_inode_getfd() has succeeded, it's impossible to undo
in a clean way and no, sys_close() is not usable in such cases.
Use anon_inode_getfile() and get_unused_fd_flags() to get struct file
and descriptor and do *not* install the file into the descriptor table
until after the last possible failure exit.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/kvm_main.c

@@ -3050,6 +3050,7 @@ static int kvm_dev_ioctl_create_vm(unsigned long type)
 {
 	int r;
 	struct kvm *kvm;
+	struct file *file;
 
 	kvm = kvm_create_vm(type);
 	if (IS_ERR(kvm))
@@ -3061,17 +3062,25 @@ static int kvm_dev_ioctl_create_vm(unsigned long type)
 		return r;
 	}
 #endif
-	r = anon_inode_getfd("kvm-vm", &kvm_vm_fops, kvm, O_RDWR | O_CLOEXEC);
+	r = get_unused_fd_flags(O_CLOEXEC);
 	if (r < 0) {
 		kvm_put_kvm(kvm);
 		return r;
 	}
+	file = anon_inode_getfile("kvm-vm", &kvm_vm_fops, kvm, O_RDWR);
+	if (IS_ERR(file)) {
+		put_unused_fd(r);
+		kvm_put_kvm(kvm);
+		return PTR_ERR(file);
+	}
 
 	if (kvm_create_vm_debugfs(kvm, r) < 0) {
-		kvm_put_kvm(kvm);
+		put_unused_fd(r);
+		fput(file);
 		return -ENOMEM;
 	}
 
+	fd_install(r, file);
 	return r;
 }