mm/mempool: avoid KASAN marking mempool poison checks as use-after-free

When removing an element from the mempool, mark it as unpoisoned in KASAN
before verifying its contents for SLUB/SLAB debugging.  Otherwise KASAN
will flag the reads checking the element use-after-free writes as
use-after-free reads.

Signed-off-by: Matthew Dawson <matthew@mjdsystems.ca>
Acked-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>