tracing: Clean up stack tracing and fix fentry updates

Akashi Takahiro was porting the stack tracer to arm64 and found some
issues with it. One was that it repeats the top function, due to the
stack frame added by the mcount caller and added by itself. This
was added when fentry came in, and before fentry created its own stack
frame. But x86's fentry now creates its own stack frame, and there's
no need to insert the function again.

This also cleans up the code a bit, where it doesn't need to do something
special for fentry, and doesn't include insertion of a duplicate
entry for the called function being traced.

Link: http://lkml.kernel.org/r/[email protected]

Some-suggestions-by: Jungseok Lee <[email protected]>
Some-suggestions-by: Mark Rutland <[email protected]>
Reported-by: AKASHI Takahiro <[email protected]>
Signed-off-by: Steven Rostedt <[email protected]>

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