CVE-2023-54167

In the Linux kernel, the following vulnerability has been resolved:

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.