tracing: Fix function graph trace_pipe to properly display failed entries

There is a case where the graph tracer might get confused and omits
displaying of a single record.  This applies mostly with the trace_pipe
since it is unlikely that the trace_seq buffer will overflow with the
trace file.

As the function_graph tracer goes through the trace entries keeping a
pointer to the current record:

current ->  func1 ENTRY
            func2 ENTRY
            func2 RETURN
            func1 RETURN

When an function ENTRY is encountered, it moves the pointer to the
next entry to check if the function is a nested or leaf function.

            func1 ENTRY
current ->  func2 ENTRY
            func2 RETURN
            func1 RETURN

If the rest of the writing of the function fills the trace_seq buffer,
then the trace_pipe read will ignore this entry. The next read will
Now start at the current location, but the first entry (func1) will
be discarded.

This patch keeps a copy of the current entry in the iterator private
storage and will keep track of when the trace_seq buffer fills. When
the trace_seq buffer fills, it will reuse the copy of the entry in the
next iteration.

[
  This patch has been largely modified by Steven Rostedt in order to
  clean it up and simplify it. The original idea and concept was from
  Jirka and for that, this patch will go under his name to give him
  the credit he deserves. But because this was modify by Steven Rostedt
  anything wrong with the patch should be blamed on Steven.
]

Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <1259067458-27143-1-git-send-email-jolsa@redhat.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>