powerpc/32: Simplify KASAN init

Since kasan_init_region() is not used anymore for modules,
KASAN init is done while slab_is_available() is false.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/84b27bf08b41c8343efd88e10f2eccd8e9f85593.1579024426.git.christophe.leroy@c-s.fr