The C++ compiler is more strict in that it refuses to assign
a void* to a struct list_head*.

Fix that by explicitly casting the poisonning constants.

(Tested with all 5 frontends, now.)
Reported-by: NRandy Dunlap <rdunlap@infradead.org>
Signed-off-by: N"Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Benjamin Poirier <bpoirier@suse.de>