KVM: let kvm_destroy_vm_debugfs clean up vCPU debugfs directories

After commit 63d04348

 ("KVM: x86: move kvm_create_vcpu_debugfs after
last failure point") we are creating the pre-vCPU debugfs files
after the creation of the vCPU file descriptor.  This makes it
possible for userspace to reach kvm_vcpu_release before
kvm_create_vcpu_debugfs has finished.  The vcpu->debugfs_dentry
then does not have any associated inode anymore, and this causes
a NULL-pointer dereference in debugfs_create_file.

The solution is simply to avoid removing the files; they are
cleaned up when the VM file descriptor is closed (and that must be
after KVM_CREATE_VCPU returns).  We can stop storing the dentry
in struct kvm_vcpu too, because it is not needed anywhere after
kvm_create_vcpu_debugfs returns.

Reported-by:  <syzbot+705f4401d5a93a59b87d@syzkaller.appspotmail.com>
Fixes: 63d04348

 ("KVM: x86: move kvm_create_vcpu_debugfs after last failure point")
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>