m68k: mm: Move initrd phys_to_virt handling after paging_init()

When booting with an initial ramdisk on platforms where physical memory
does not start at address zero (e.g. on Amiga):

    initrd: 0ef0602c - 0f800000
    Zone ranges:
      DMA      [mem 0x0000000008000000-0x000000f7ffffffff]
      Normal   empty
    Movable zone start for each node
    Early memory node ranges
      node   0: [mem 0x0000000008000000-0x000000000f7fffff]
    Initmem setup node 0 [mem 0x0000000008000000-0x000000000f7fffff]
    Unable to handle kernel access at virtual address (ptrval)
    Oops: 00000000
    Modules linked in:
    PC: [<00201d3c>] memcmp+0x28/0x56

As phys_to_virt() relies on m68k_memoffset and module_fixup(), it must
not be called before paging_init().  Hence postpone the phys_to_virt
handling for the initial ramdisk until after calling paging_init().

While at it, reduce #ifdef clutter by using IS_ENABLED() instead.

Fixes: 376e3fd ("m68k: Enable memtest functionality")
Reported-by: Stephen Walsh <vk3heg@vk3heg.net>
Link: https://lists.debian.org/debian-68k/2022/09/msg00007.html
Reported-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Link: https://lore.kernel.org/r/4f45f05f377bf3f5baf88dbd5c3c8aeac59d94f0.camel@physik.fu-berlin.de
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Finn Thain <fthain@linux-m68k.org>
Link: https://lore.kernel.org/r/dff216da09ab7a60217c3fc2147e671ae07d636f.1677528627.git.geert@linux-m68k.org

arch/m68k/kernel/setup_mm.c

@@ -326,16 +326,16 @@ void __init setup_arch(char **cmdline_p)
 		panic("No configuration setup");
 	}
 
-#ifdef CONFIG_BLK_DEV_INITRD
-	if (m68k_ramdisk.size) {
+	if (IS_ENABLED(CONFIG_BLK_DEV_INITRD) && m68k_ramdisk.size)
 		memblock_reserve(m68k_ramdisk.addr, m68k_ramdisk.size);
+
+	paging_init();
+
+	if (IS_ENABLED(CONFIG_BLK_DEV_INITRD) && m68k_ramdisk.size) {
 		initrd_start = (unsigned long)phys_to_virt(m68k_ramdisk.addr);
 		initrd_end = initrd_start + m68k_ramdisk.size;
 		pr_info("initrd: %08lx - %08lx\n", initrd_start, initrd_end);
 	}
-#endif
-
-	paging_init();
 
 #ifdef CONFIG_NATFEAT
 	nf_init();