ext4: fix WARN_ON from ext4_releasepage()

ext4_releasepage() warns when it is passed a page with PageChecked set.
However this can correctly happen when invalidate_inode_pages2_range()
invalidates pages - and we should fail the release in that case. Since
the page was dirty anyway, it won't be discarded and no harm has
happened but it's good to be safe. Also remove bogus page_has_buffers()
check - we are guaranteed page has buffers in this function.
Reported-by: NZheng Liu <gnehzuil.liu@gmail.com>
Tested-by: NZheng Liu <wenqing.lz@taobao.com>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: NJan Kara <jack@suse.cz>

fs/ext4/inode.c

@@ -3018,8 +3018,8 @@ static int ext4_releasepage(struct page *page, gfp_t wait)
 
 	trace_ext4_releasepage(page);
 
-	WARN_ON(PageChecked(page));
-	if (!page_has_buffers(page))
+	/* Page has dirty journalled data -> cannot release */
+	if (PageChecked(page))
 		return 0;
 	if (journal)
 		return jbd2_journal_try_to_free_buffers(journal, page, wait);