ftrace: fix dyn ftrace filter

Impact: correct implementation of dyn ftrace filter

The old decisions made by the filter algorithm was complex and incorrect.
This lead to inconsistent enabling or disabling of functions when
the filter was used.

This patch simplifies that code and in doing so, corrects the usage
of the filters.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>