m68k: Enable memtest functionality

Enable the memtest functionality and rearrange some code to prevent it
from clobbering the initrd.

The code to implement CONFIG_BLK_DEV_INITRD was conditional on
!defined(CONFIG_SUN3). For simplicity, remove that test on the basis
that m68k_ramdisk.size == 0 on Sun 3. The SLIME source code at
http://sammy.net/sun3/ftp/pub/m68k/sun3/slime/slime-2.0.tar.gz


indicates that no BI_RAMDISK entry is ever passed to the kernel due
to #ifdef 0 around the relevant code.

Cc: Mike Rapoport <rppt@kernel.org>
Cc: Sam Creasey <sammy@sammy.net>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Finn Thain <fthain@linux-m68k.org>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Link: https://lore.kernel.org/r/8170fe1d1c62426d82275d36ba409ecc18754292.1637274578.git.fthain@linux-m68k.org


Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>