writeback instrumentation: remove duplicated structures

Don't replicate internal structures from the kernel: this is asking for
serious trouble, and could lead to breakage if building on newer kernels
that have modified structures.

The proper approach, if we really need to extract this information,
would be to add APIs to the Linux kernel workqueue that allow getting
this information.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>