linux-yocto/4.8: fix BUG_ON() in workingset_node_shadows_dec() triggers

linux-yocto/4.8: fix BUG_ON() in workingset_node_shadows_dec() triggers

Paul Gotmaker pointed out that a last minute merge to the 4.8 kernel
has the potential to hard hang a kernel when VM debugging is enabled:

  https://lkml.org/lkml/2016/10/4/1

He also pointed out the fix for it in commit 21f54dda
[Using BUG_ON() as an assert() is _never_ acceptable].

While that fix will loop through -stable into 4.8.1, that will
likely be too late for our release. So I've cherry picked the
change to make it available.

(From OE-Core rev: eb4b39d5ffbe93d363b05c57196bdac61fa09c59)

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>