do_migrate_range() returns migrate_pages() return value, which zero
means perfect success, in usual cases. If all pages are failed to be
isolated, however, it returns isolate_{lru,movalbe}_page() return
values, or zero if all pfn were invalid, were hugetlb or hwpoisoned. So
do_migrate_range() returning zero means either perfect success, or
special cases of isolation total failure.
Actually, the return value is not checked by any caller, so it might be
better to simply make it a void function. However, there is a TODO for
checking the return value.