Gcc warns that addr might be used uninitialized. It may not, but I see
why gcc gets confused.

Additionally, hiding code with side-effects inside WARN_ON() argument
seems uncool, so I moved it outside.
Signed-off-by: NPavel Machek <pavel@ucw.cz>
[danvet: Add obligatory /* shuts up gcc */ comment.]
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>