perf ftrace: Use ftrace_ops::private to store event pointer

Having following commands running concurently:

  # perf record -e ftrace:function -a -o krava.data sleep 10
  # perf record -e ftrace:function --filter 'ip == SyS_read' ls

will endup in the latter one failing on the filter
rules and store all functions (in perf.data) as
instructed by the first record instead of just
SyS_read records.

The reason is that we don't check the ftrace_ops that
triggered the event with event's ftrace_ops. Hence
once running together the event from latter perf will
get all the data of the event from the first one.

Fixing this by using ftrace_ops::private value to keep
the perf_event pointer. This way we don't need to search
for triggered event (as tracepoint handler does) and
directly store sample.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>