perf arm-spe: Refactor arm_spe_get_events()

perf arm-spe: Refactor arm_spe_get_events()

In function arm_spe_get_events(), the event packet's 'index' is assigned
as payload length, but the flow is not directive: it firstly gets the
packet length from the return value of arm_spe_get_payload(), the value
includes header length (1) and payload length:

  int ret = arm_spe_get_payload(buf, len, packet);

and then reduces header length from packet length, so finally get the
payload length:

  packet->index = ret - 1;

To simplify the code, this patch directly assigns payload length to
event packet's index; and at the end it calls arm_spe_get_payload() to
return the payload value.

Signed-off-by: Leo Yan <leo.yan@linaro.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Link: https://lore.kernel.org/r/20201111071149.815-5-leo.yan@linaro.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>