ftrace: Update func_pos in t_start() when all functions are enabled

If all functions are enabled, there's a comment displayed in the file to
denote that:

  # cd /sys/kernel/debug/tracing
  # cat set_ftrace_filter
 #### all functions enabled ####

If a function trigger is set, those are displayed as well:

  # echo schedule:traceoff >> /debug/tracing/set_ftrace_filter
  # cat set_ftrace_filter
 #### all functions enabled ####
 schedule:traceoff:unlimited

But if you read that file with dd, the output can change:

  # dd if=/debug/tracing/set_ftrace_filter bs=1
 #### all functions enabled ####
 32+0 records in
 32+0 records out
 32 bytes copied, 7.0237e-05 s, 456 kB/s

This is because the "pos" variable is updated for the comment, but func_pos
is not. "func_pos" is used by the triggers (or hashes) to know how many
functions were printed and it bases its index from the pos - func_pos.
func_pos should be 1 to count for the comment printed. But since it is not,
t_hash_start() thinks that one trigger was already printed.

The cat gets to t_hash_start() via t_next() and not t_start() which updates
both pos and func_pos.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/ftrace.c

@@ -3230,6 +3230,7 @@ static void *t_start(struct seq_file *m, loff_t *pos)
 	 */
 	if ((iter->flags & (FTRACE_ITER_FILTER | FTRACE_ITER_NOTRACE)) &&
 	    ftrace_hash_empty(iter->hash)) {
+		iter->func_pos = 1; /* Account for the message */
 		if (*pos > 0)
 			return t_hash_start(m, pos);
 		iter->flags |= FTRACE_ITER_PRINTALL;