tracing: Fix possible NULL pointer dereferences

Currently set_ftrace_pid and set_graph_function files use seq_lseek
for their fops.  However seq_open() is called only for FMODE_READ in
the fops->open() so that if an user tries to seek one of those file
when she open it for writing, it sees NULL seq_file and then panic.

It can be easily reproduced with following command:

  $ cd /sys/kernel/debug/tracing
  $ echo 1234 | sudo tee -a set_ftrace_pid

In this example, GNU coreutils' tee opens the file with fopen(, "a")
and then the fopen() internally calls lseek().

Link: http://lkml.kernel.org/r/1365663302-2170-1-git-send-email-namhyung@kernel.org

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Namhyung Kim <namhyung.kim@lge.com>
Cc: stable@vger.kernel.org
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
t_hash_show(struct seq_file *m, struct ftrace_iterator *iter)
{
	struct ftrace_func_probe *rec;

	rec = iter->probe;
	if (WARN_ON_ONCE(!rec))
		return -EIO;

	if (rec->ops->print)
		return rec->ops->print(m, rec->ip, rec->ops, rec->data);

	seq_printf(m, "%ps:%ps", (void *)rec->ip, (void *)rec->ops->func);

	if (rec->data)
		seq_printf(m, ":%p", rec->data);
	seq_putc(m, '\n');

	return 0;
}

t_hash_show(struct seq_file *m, struct ftrace_iterator *iter)
{
	struct ftrace_func_probe *rec;

	rec = iter->probe;
	if (WARN_ON_ONCE(!rec))
		return -EIO;

	if (rec->ops->print)
		return rec->ops->print(m, rec->ip, rec->ops, rec->data);

	seq_printf(m, "%ps:%ps", (void *)rec->ip, (void *)rec->ops->func);

	if (rec->data)
		seq_printf(m, ":%p", rec->data);
	seq_putc(m, '\n');

	return 0;
}
