ftrace: restore iterator trace in pipe read

The trace iterator is reset in the read. We still need to restore the tracer
that the trace_pipe was opened with.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>