x86/ftrace: Add comment on static function tracing

There was a confusion between update_ftrace_function() and static
function tracing trampoline regarding 3rd parameter (ftrace_ops).
Add a comment for clarification.

Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: H. Peter Anvin <hpa@linux.intel.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/1447721004-2551-1-git-send-email-namhyung@kernel.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/mcount_64.S

@@ -278,6 +278,12 @@ trace:
 	/* save_mcount_regs fills in first two parameters */
 	save_mcount_regs
 
+	/*
+	 * When DYNAMIC_FTRACE is not defined, ARCH_SUPPORTS_FTRACE_OPS is not
+	 * set (see include/asm/ftrace.h and include/linux/ftrace.h).  Only the
+	 * ip and parent ip are used and the list function is called when
+	 * function tracing is enabled.
+	 */
 	call   *ftrace_trace_function
 
 	restore_mcount_regs