ftrace: Clean up __seq_open_private() return check

The return status check of __seq_open_private() is rather strange:

	iter = __seq_open_private();
	if (iter) {
		/* do stuff */
	}

	return iter ? 0 : -ENOMEM;

It makes much more sense to do the return of failure right away:

	iter = __seq_open_private();
	if (!iter)
		return -ENOMEM;

	/* do stuff */

	return 0;

This clean up will make updates to this code a bit nicer.

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