linux.core: Simplify KernelThreadInformationProvider

linux.core: Simplify KernelThreadInformationProvider

This makes KernelthreadInformationProvider only check pertinent state
values. The logic should be 100% equivalent, but the code should be
much more legible.

Change-Id: I96a962c6f2fa80b2a57d753a324c60c394dae349
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/69975
Reviewed-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Tested-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Reviewed-by: Hudson CI