So what did I learn today?
When the Linux kernel documentation says about a function "this will modify the original iovec", it will indeed modify the original iovec. And it will modify it in a way you are not anticipating, so any attempt to undo this modification is futile and will fail horribly - I eventually stopped counting the number of hard hangups of my test system.
When the Linux kernel documentation says about a function "this will modify the original iovec", it will indeed modify the original iovec. And it will modify it in a way you are not anticipating, so any attempt to undo this modification is futile and will fail horribly - I eventually stopped counting the number of hard hangups of my test system.