powerpc/32s: Use dedicated segment for modules with STRICT_KERNEL_RWX

When STRICT_KERNEL_RWX is set, we want to set NX bit on vmalloc
segments. But modules require exec.

Use a dedicated segment for modules. There is not much space
above kernel, and we don't waste vmalloc space to do alignment.
Therefore, we take the segment before PAGE_OFFSET for modules.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/eb8faba9148b6cf17c696ba776b4e8ee2f6313bf.1593428200.git.christophe.leroy@csgroup.eu