mm/memory_hotplug: cleanup return value handing in do_migrate_range()

Return value mechanism of do_migrate_range() is not very simple, while no
caller of the function checks the return value.  Make the function return
nothing to be more simple, and cleanup related unnecessary code.

Link: https://lkml.kernel.org/r/20230216170703.64574-1-sj@kernel.org


Suggested-by: David Hildenbrand <david@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>