powerpc/32s: Fix is_module_segment() when MODULES_VADDR is defined

When MODULES_VADDR is defined, is_module_segment() shall check the
address against it instead of checking agains VMALLOC_START.

Fixes: 6ca05532 ("powerpc/32s: Use dedicated segment for modules with STRICT_KERNEL_RWX")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/07884ed033c31e074747b7eb8eaa329d15db07ec.1596641219.git.christophe.leroy@csgroup.eu