tracing: Check the return value of trace_get_user()

Return immediately if trace_get_user() returned failure.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <4AB86614.7020803@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>