trace-cmd: Reset ftrace pid on disable all
trace-cmd: Reset ftrace pid on disable all

Reset the ftrace pid on disable all and also fix the reset of the
file to perform a O_TRUNC, otherwise it keeps the previous pids.

Signed-off-by: Steven Rostedt <43232e92d70cc7aa53504ad0397085ee47bad87f@goodmis.org>

bigzz/trace-cmd,bigzz/trace-cmd