s390/mm: uninline copy_oldmem_kernel() function

Uninline copy_oldmem_kernel() function and make it consistent
with a very similar memcpy_real() implementation, by moving
to code to crash_dump.c, where it actually belongs.

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>