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: 376e3fde ("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