powerpc/32: Force KASAN_VMALLOC for modules

Unloading/Reloading of modules seems to fail with KASAN_VMALLOC
but works properly with it.

Force selection of KASAN_VMALLOC when MODULES are selected, and
drop module_alloc() which was dedicated to KASAN for modules.

Reported-by:  <erhard_f@mailbox.org>
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=205283
Link: https://lore.kernel.org/r/f909da11aecb59ab7f32ba01fae6f356eaa4d7bc.1579024426.git.christophe.leroy@c-s.fr