powerpc: Don't provide __kernel_map_pages() without ARCH_SUPPORTS_DEBUG_PAGEALLOC

When ARCH_SUPPORTS_DEBUG_PAGEALLOC is not selected, the user can
still select CONFIG_DEBUG_PAGEALLOC in which case __kernel_map_pages()
is provided by mm/page_poison.c

So only define __kernel_map_pages() when both
CONFIG_ARCH_SUPPORTS_DEBUG_PAGEALLOC and CONFIG_DEBUG_PAGEALLOC
are defined.

Fixes: 68b44f94d6 ("powerpc/booke: Disable STRICT_KERNEL_RWX, DEBUG_PAGEALLOC and KFENCE")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/971b69739ff4746252e711a9845210465c023a9e.1635425947.git.christophe.leroy@csgroup.eu

arch/powerpc/mm/pgtable_32.c

@@ -173,7 +173,7 @@ void mark_rodata_ro(void)
 }
 #endif
 
-#ifdef CONFIG_DEBUG_PAGEALLOC
+#if defined(CONFIG_ARCH_SUPPORTS_DEBUG_PAGEALLOC) && defined(CONFIG_DEBUG_PAGEALLOC)
 void __kernel_map_pages(struct page *page, int numpages, int enable)
 {
 	unsigned long addr = (unsigned long)page_address(page);