Reiser4 can't be built as a module when EXPORT_UNUSED_SYMBOL is not set.

It's broken because reiser4_write_extent is calling
simple_prepare_write() and commit
4e02ed4b scheduled it for unexporting.
Signed-off-by: Laurent Riffard <laurent.riffard@free.fr>
Acked-by: Edward Shishkin <edward.shishkin@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/reiser4/plugin/item/extent_file_ops.c

@@ -1059,8 +1059,9 @@ ssize_t reiser4_write_extent(struct file *file, struct inode * inode,
 
 		lock_page(page);
 		if (!PageUptodate(page) && to_page != PAGE_CACHE_SIZE)
-			simple_prepare_write(file, page, page_off,
-					     page_off + to_page);
+			zero_user_segments(page, 0, page_off,
+			                   page_off + to_page,
+			                   PAGE_CACHE_SIZE);
 
 		written = filemap_copy_from_user(page, page_off, buf, to_page);
 		if (unlikely(written != to_page)) {