UPSTREAM: tracing: Use cpumask_available() to check if cpumask variable may be used

UPSTREAM: tracing: Use cpumask_available() to check if cpumask variable may be used

This fixes the following clang warning:

kernel/trace/trace.c:3231:12: warning: address of array 'iter->started'
  will always evaluate to 'true' [-Wpointer-bool-conversion]
        if (iter->started)

Link: http://lkml.kernel.org/r/20170421234110.117075-1-mka@chromium.org
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
(cherry picked from commit 4dbbe2d8e95c351157f292ece067f985c30c7b53)
Bug: 78886293
Change-Id: Ib17a68ce55ca80b04bdea2d232f8ca9f88b1b8a3
Signed-off-by: Alistair Strachan <astrachan@google.com>