Ideally we shall have tests for all possible races, that is races
between if_detach(), if_vmove_loan(), if_vmove_reclaim() and
vnet_if_return(). But that requires too many tests and it appears to
be less valuable to have them all. So focus on recent fixes [1] and [2].
[1] ee9456ce3753 ifnet: Fix races in if_vmove_reclaim()
[2] D56374 ifnet: if_detach(): Fix races with if_vmove()
MFC after: 5 days