tracing: Use __this_cpu_read() in trace_buffered_event_enable()

The code is executed with preemption disabled, so it's
safe to use __this_cpu_read().

Link: https://lkml.kernel.org/r/20200813112803.12256-1-tian.xianting@h3c.com

Signed-off-by: Xianting Tian <tian.xianting@h3c.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>