initrd: Fix virtual/physical mix-up in overwrite test

On recent kernels, I get the following error when using an initrd:

| initrd overwritten (0x00b78000 < 0x07668000) - disabling it.

My Amiga 4000 has 12 MiB of RAM at physical address 0x07400000 (virtual
0x00000000).
The initrd is located at the end of RAM: 0x00b78000 - 0x00c00000 (virtual).
The overwrite test compares the (virtual) initrd location to the (physical)
first available memory location, which fails.

This patch converts initrd_start to a page frame number, so it can safely be
compared with min_low_pfn.

Before the introduction of discontiguous memory support on m68k
(12d810c1

), min_low_pfn was just left
untouched by the m68k-specific code (zero, I guess), and everything worked
fine.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>