UPSTREAM: coresight: tmc-etf: Fix snapshot mode update function

(Upstream commit 99f81eb9c51d499f65bac25597e9def22f6a32dc).

When working in snapshot mode function perf_aux_output_begin()
does not set the handle->size because the size is expected to be
deduced by the placement of the "head" and "old" pointers in user
space.  As such there is no point in trying to adjust the amount
of data to copy to the ring buffer.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Reviewed-by: Leo Yan <leo.yan@linaro.org>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Tested-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Bug: 140266694
Change-Id: I58ca0bb839b3da1d337517c1f5c00583deac4245
Signed-off-by: Yabin Cui <yabinc@google.com>