powerpc/kasan: Fix shadow start address with modules

Modules are now located before kernel, KASAN area has to
be extended accordingly.

Fixes: 80edc68e ("powerpc/32s: Define a MODULE area below kernel text all the time")
Fixes: 9132a2e8 ("powerpc/8xx: Define a MODULE area below kernel text")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/c68163065163f303f5af1e4bbdd9f1ce69f0543e.1619260465.git.christophe.leroy@csgroup.eu