s390/mem_detect: rely on diag260() if sclp_early_get_memsize() fails

In case sclp_early_get_memsize() fails but diag260() succeeds make sure
some sane value is returned. This error scenario is highly unlikely,
but this change makes system able to boot in such case.
Suggested-by: Alexander Gordeev <agordeev@linux.ibm.com>
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/boot/mem_detect.c

@@ -176,7 +176,7 @@ unsigned long detect_memory(void)
 
 	if (!diag260()) {
 		mem_detect.info_source = MEM_DETECT_DIAG260;
-		return max_physmem_end;
+		return max_physmem_end ?: get_mem_detect_end();
 	}
 
 	if (max_physmem_end) {