bug: set warn variable before calling WARN()

This has hit me a couple of times already.  I would be debugging code
and the system would simply hang and then reboot.  Finally, I found that
the problem was caused by WARN_ON_ONCE() and friends.

The macro WARN_ON_ONCE(condition) is defined as:

	static bool __section(.data.unlikely) __warned;
	int __ret_warn_once = !!(condition);

	if (unlikely(__ret_warn_once))
		if (WARN_ON(!__warned))
			__warned = true;

	unlikely(__ret_warn_once);

Which looks great and all.  But what I have hit, is an issue when
WARN_ON() itself hits the same WARN_ON_ONCE() code.  Because, the
variable __warned is not yet set.  Then it too calls WARN_ON() and that
triggers the warning again.  It keeps doing this until the stack is
overflowed and the system crashes.

By setting __warned first before calling WARN_ON() makes the original
WARN_ON_ONCE() really only warn once, and not an infinite amount of
times if the WARN_ON() also triggers the warning.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>