When a page fault occurs on a file-backed mapping for a page that
was previously in the page cache but has later been evicted, the
fault handler calls pagecache_map_page_if_filled(), which should
return `false` because the newly re-allocated page has not been
filled yet. Commit 132a832
introduced a regression which causes pagecache_map_page_if_filled()
to return `true` instead; this leads to page cache contents
inconsistent with file contents and erroneous data accessed by the
user program via the file-backed mapping.
This change fixes the above issue by amending the
touch_or_fill_page_nodelocked() function so that it returns `false`
when the relevant page is in the ALLOC state.