lib: stackinit: Adjust target string to 8 bytes for m68k

For reasons I cannot understand, m68k moves the start of the stack frame
for consecutive calls to the same function if the function's test
variable is larger than 8 bytes. This was only happening for the char
array test (obviously), so adjust the length of the string for m68k
only. I want the array size to be longer than "unsigned long" for every
given architecture, so the other remain unchanged.

Additionally adjust the error message to be a bit more clear about
what's happened, and move the KUNIT check outside of the consecutive
calls to minimize what happens between them.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Closes: https://lore.kernel.org/lkml/a0d10d50-2720-4ecd-a2c6-c2c5e5aeee65@roeck-us.net/


Tested-by: Guenter Roeck <linux@roeck-us.net>
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Closes: https://lore.kernel.org/r/CAMuHMdX_g1tbiUL9PUQdqaegrEzCNN3GtbSvSBFYAL4TzvstFg@mail.gmail.com
Closes: https://lore.kernel.org/r/CAMuHMdW6N40+0gGQ+LSrN64Mo4A0-ELAm0pR3gWQ0mNanyBuUQ@mail.gmail.com


Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/all/a4bf4063-194f-4740-9c1d-88f9ab38b778@roeck-us.net


Signed-off-by: Kees Cook <keescook@chromium.org>