xfs: Fix WARN_ON(delalloc) in xfs_vm_releasepage()

When a dirty page is truncated from a file but reclaim gets to it before
truncate_inode_pages(), we hit WARN_ON(delalloc) in
xfs_vm_releasepage(). This is because reclaim tries to write the page,
xfs_vm_writepage() just bails out (leaving page clean) and thus reclaim
thinks it can continue and calls xfs_vm_releasepage() on page with dirty
buffers.

Fix the issue by redirtying the page in xfs_vm_writepage(). This makes
reclaim stop reclaiming the page and also logically it keeps page in a
more consistent state where page with dirty buffers has PageDirty set.
Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

fs/xfs/xfs_aops.c

@@ -953,13 +953,13 @@ xfs_vm_writepage(
 		unsigned offset_into_page = offset & (PAGE_CACHE_SIZE - 1);
 
 		/*
-		 * Just skip the page if it is fully outside i_size, e.g. due
-		 * to a truncate operation that is in progress.
+		 * Skip the page if it is fully outside i_size, e.g. due to a
+		 * truncate operation that is in progress. We must redirty the
+		 * page so that reclaim stops reclaiming it. Otherwise
+		 * xfs_vm_releasepage() is called on it and gets confused.
 		 */
-		if (page->index >= end_index + 1 || offset_into_page == 0) {
-			unlock_page(page);
-			return 0;
-		}
+		if (page->index >= end_index + 1 || offset_into_page == 0)
+			goto redirty;
 
 		/*
 		 * The page straddles i_size.  It must be zeroed out on each