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: df9694c ("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>

arch/s390/kernel/crash_dump.c

@@ -233,9 +233,10 @@ ssize_t copy_oldmem_page(struct iov_iter *iter, unsigned long pfn, size_t csize,
 		rc = copy_oldmem_user(iter->iov->iov_base, src, csize);
 	else
 		rc = copy_oldmem_kernel(iter->kvec->iov_base, src, csize);
-	if (!rc)
-		iov_iter_advance(iter, csize);
-	return rc;
+	if (rc < 0)
+		return rc;
+	iov_iter_advance(iter, csize);
+	return csize;
 }
 
 /*