tracing: Move pid_list write processing into its own function

The addition of PIDs into a pid_list via the write operation of
set_event_pid is a bit complex. The same operation will be needed for
function tracing pids. Move the code into its own generic function in
trace.c, so that we can avoid duplication of this code.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>