Make __stringify support variable argument macros too

For example:

  __stringify(__entry->irq, __entry->ret)

will now convert it to:

  "REC->irq, REC->ret"

It also still supports single arguments as the old macro did.

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <49DC6751.30308@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>