powerpc/kasan: Fix parallel loading of modules.

Parallel loading of modules may lead to bad setup of shadow page table
entries.

First, lets align modules so that two modules never share the same
shadow page.

Second, ensure that two modules cannot allocate two page tables for
the same PMD entry at the same time. This is done by using
init_mm.page_table_lock in the same way as __pte_alloc_kernel()

Fixes: 2edb16ef

 ("powerpc/32: Add KASAN support")
Cc: stable@vger.kernel.org # v5.2+
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/c97284f912128cbc3f2fe09d68e90e65fb3e6026.1565361876.git.christophe.leroy@c-s.fr