Get rid of linux-core-thread-data

This struct type seems to have been used in the past as a callback
parameter.  Now it seems that case is no longer true, so we can simplify
things by passing the individual parameters linux_core_thread_data
encapsulates directly to the functions.

This is just a cleanup before the next change.

Approved-By: Simon Marchi <simon.marchi@efficios.com>