migrate_pages(): fix failure counting for THP on -ENOSYS

If THP or hugetlbfs page migration isn't supported, unmap_and_move() or
unmap_and_move_huge_page() will return -ENOSYS.  For THP, splitting will
be tried, but if splitting doesn't succeed, the THP will be left in "from"
list wrongly.  If some other pages are retried, the THP migration failure
will counted again.  This is fixed via moving the failure THP from "from"
to "ret_pages".

Another issue of the original code is that the unsupported failure
processing isn't consistent between THP and hugetlbfs page.  Make them
consistent in this patch to make the code easier to be understood too.

Link: https://lkml.kernel.org/r/20220817081408.513338-6-ying.huang@intel.com


Fixes: 5984fabb ("mm: move_pages: report the number of non-attempted pages")
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>