l2tpv3: fix fd leak

In this false branch, fd will leak when it is zero.
Change the testing condition.

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
[Fix net_l2tpv3_cleanup as well. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
