Make sure that collapse_file doesn't interfere with checking the uptodate
flag in the page cache by only inserting hpage into the page cache after
it has been updated and marked uptodate.  This is achieved by simply not
replacing present pages with hpage when iterating over the target range.

The present pages are already locked, so replacing them with the locked
hpage before the collapse is finalized is unnecessary.  However, it is
necessary to stop freezing the present pages after validating them, since
leaving long-term frozen pages in the page cache can lead to deadlocks. 
Simply checking the reference count is sufficient to ensure that there are
no long-term references hanging around that would the collapse would
break.  Similar to hpage, there is no reason that the present pages
actually need to be frozen in addition to being locked.

This fixes a race where folio_seek_hole_data would mistake hpage for an
fallocated but unwritten page.  This race is visible to userspace via data
temporarily disappearing from SEEK_DATA/SEEK_HOLE.  This also fixes a
similar race where pages could temporarily disappear from mincore.

Link: https://lkml.kernel.org/r/20230404120117.2562166-5-stevensd@google.com
Fixes: f3f0e1d2 ("khugepaged: add support of collapse for tmpfs/shmem pages")
Signed-off-by: David Stevens <stevensd@chromium.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jiaqi Yan <jiaqiyan@google.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Yang Shi <shy828301@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>