s390/crash: make copy_oldmem_page() return number of bytes copied

Callback copy_oldmem_page() returns either error code or zero.
Instead, it should return the error code or number of bytes copied.

Fixes: df9694c7 ("s390/dump: streamline oldmem copy functions")
Reviewed-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Tested-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>