ftrace: fix set_ftrace_filter

Impact: fix of output of set_ftrace_filter

The commit "ftrace: do not show freed records in
             available_filter_functions"

Removed a bit too much from the set_ftrace_filter code, where we now see
all functions in the set_ftrace_filter file even when we set a filter.

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