tracing/events: fix output format of kernel stack

According to "events/ftrace/kernel_stack/format", output format of
kernel stack should use "=>" instead of "<=".

The second problem is that we shouldn't skip the first entry in the stack,
although it seems to be duplicated when used in the "function" tracer,
but events also use it. If we skip the first one, we will drop the topmost
entry of the stack.

The last problem is that if the last entry is ULONG_MAX(0xffffffff), we should
drop it, otherwise it will print a NULL name line.

before fix:

      sh-1072  [000]   26.957239: sched_process_fork: parent sh:1072 child sh:1073
      sh-1072  [000]   26.957262:
 <= syscall_call
 <=
      sh-1072  [000]   26.957744: sched_switch: task sh:1072 [120] (R) ==> sh:1073 [120]
      sh-1072  [000]   26.957752:
 <= preempt_schedule
 <= wake_up_new_task
 <= do_fork
 <= sys_clone
 <= syscall_call
 <=

After fix:

      sh-1075  [000]    39.791848: sched_process_fork: parent sh:1075  child sh:1076
      sh-1075  [000]    39.791871:
 => sys_clone
 => syscall_call
      sh-1075  [000]    39.792713: sched_switch: task sh:1075 [120] (R) ==> sh:1076 [120]
      sh-1075  [000]    39.792722:
 => schedule
 => preempt_schedule
 => wake_up_new_task
 => do_fork
 => sys_clone
 => syscall_call
Signed-off-by: Nwalimis <walimisdev@gmail.com>
LKML-Reference: <1244016090-7814-2-git-send-email-walimisdev@gmail.com>
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>

kernel/trace/trace_output.c

@@ -975,16 +975,16 @@ static enum print_line_t trace_stack_print(struct trace_iterator *iter,
 
 	trace_assign_type(field, iter->ent);
 
+	if (!trace_seq_puts(s, "\n"))
+		goto partial;
 	for (i = 0; i < FTRACE_STACK_ENTRIES; i++) {
-		if (!field->caller[i])
+		if (!field->caller[i] || (field->caller[i] == ULONG_MAX))
 			break;
-		if (i) {
-			if (!trace_seq_puts(s, " <= "))
-				goto partial;
+		if (!trace_seq_puts(s, " => "))
+			goto partial;
 
-			if (!seq_print_ip_sym(s, field->caller[i], flags))
-				goto partial;
-		}
+		if (!seq_print_ip_sym(s, field->caller[i], flags))
+			goto partial;
 		if (!trace_seq_puts(s, "\n"))
 			goto partial;
 	}