KVM: explain missing kvm_put_kvm in case of failure

The call to kvm_put_kvm was removed from error handling in commit
506cfba9e7 ("KVM: don't use anon_inode_getfd() before possible
failures"), but it is _not_ a memory leak.  Reuse Al's explanation
to avoid that someone else makes the same mistake.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/kvm_main.c

@@ -3191,6 +3191,12 @@ static int kvm_dev_ioctl_create_vm(unsigned long type)
 		return PTR_ERR(file);
 	}
 
+	/*
+	 * Don't call kvm_put_kvm anymore at this point; file->f_op is
+	 * already set, with ->release() being kvm_vm_release().  In error
+	 * cases it will be called by the final fput(file) and will take
+	 * care of doing kvm_put_kvm(kvm).
+	 */
 	if (kvm_create_vm_debugfs(kvm, r) < 0) {
 		put_unused_fd(r);
 		fput(file);