stacktrace: provide save_stack_trace_tsk() weak alias

Impact: build fix

Some architectures have not implemented save_stack_trace_tsk() yet:

  fs/built-in.o: In function `proc_pid_stack':
  base.c:(.text+0x3f140): undefined reference to `save_stack_trace_tsk'

So warn about that if the facility is used.
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/stacktrace.c

@@ -6,6 +6,7 @@
  *  Copyright (C) 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
  */
 #include <linux/sched.h>
+#include <linux/kernel.h>
 #include <linux/module.h>
 #include <linux/kallsyms.h>
 #include <linux/stacktrace.h>
@@ -24,3 +25,13 @@ void print_stack_trace(struct stack_trace *trace, int spaces)
 }
 EXPORT_SYMBOL_GPL(print_stack_trace);
 
+/*
+ * Architectures that do not implement save_stack_trace_tsk get this
+ * weak alias and a once-per-bootup warning (whenever this facility
+ * is utilized - for example by procfs):
+ */
+__weak void
+save_stack_trace_tsk(struct task_struct *tsk, struct stack_trace *trace)
+{
+	WARN_ONCE(1, KERN_INFO "save_stack_trace_tsk() not implemented yet.\n");
+}