tracing: Add rcu annotation for syscall trace descriptors

sparse complains about the enter/exit_sysycall_files[] variables being
dereferenced with rcu_dereference_sched(). The fields need to be
annotated with __rcu.
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace.h

@@ -192,8 +192,8 @@ struct trace_array {
 #ifdef CONFIG_FTRACE_SYSCALLS
 	int			sys_refcount_enter;
 	int			sys_refcount_exit;
-	struct ftrace_event_file *enter_syscall_files[NR_syscalls];
-	struct ftrace_event_file *exit_syscall_files[NR_syscalls];
+	struct ftrace_event_file __rcu *enter_syscall_files[NR_syscalls];
+	struct ftrace_event_file __rcu *exit_syscall_files[NR_syscalls];
 #endif
 	int			stop_count;
 	int			clock_id;