mm/khugepaged: fix conflicting mods to collapse_file()

Inserting Ivan Orlov's syzbot fix commit 2ce0bdfe
("mm: khugepaged: fix kernel BUG in hpage_collapse_scan_file()")
ahead of Jiaqi Yan's and David Stevens's commits
12904d95 ("mm/khugepaged: recover from poisoned file-backed memory")
cae106dd ("mm/khugepaged: refactor collapse_file control flow")
ac492b9c ("mm/khugepaged: skip shmem with userfaultfd")
(all of which restructure collapse_file()) did not work out well.

xfstests generic/086 on huge tmpfs (with accelerated khugepaged) freezes
(if not on the first attempt, then the 2nd or 3rd) in find_lock_entries()
while doing drop_caches: the file's xarray seems to have been corrupted,
with find_get_entry() returning nonsense which makes no progress.

Bisection led to ac492b9c; and diff against earlier working linux-next
suggested that it's probably down to an errant xas_store(), which does not
belong with the later changes (and nor does the positioning of warnings). 
The later changes look as if they fix the syzbot issue independently.

Remove most of what's left of 2ce0bdfe

: just leave one WARN_ON_ONCE
(xas_error) after the final xas_store() of the multi-index entry.

Link: https://lkml.kernel.org/r/b6c881-c352-bb91-85a8-febeb09dfd71@google.com
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: David Stevens <stevensd@chromium.org>
Cc: Ivan Orlov <ivan.orlov0322@gmail.com>
Cc: Jiaqi Yan <jiaqiyan@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>