ext3: add writepage sanity checks

- There is theoretical possibility to perform writepage on
   RO superblock. Add explicit check for what case.
- Page must being locked before writepage.

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/ext3/inode.c

@@ -1528,6 +1528,7 @@ static int ext3_ordered_writepage(struct page *page,
 	int err;
 
 	J_ASSERT(PageLocked(page));
+	WARN_ON_ONCE(IS_RDONLY(inode));
 
 	/*
 	 * We give up here if we're reentered, because it might be for a
@@ -1600,6 +1601,9 @@ static int ext3_writeback_writepage(struct page *page,
 	int ret = 0;
 	int err;
 
+	J_ASSERT(PageLocked(page));
+	WARN_ON_ONCE(IS_RDONLY(inode));
+
 	if (ext3_journal_current_handle())
 		goto out_fail;
 
@@ -1642,6 +1646,9 @@ static int ext3_journalled_writepage(struct page *page,
 	int ret = 0;
 	int err;
 
+	J_ASSERT(PageLocked(page));
+	WARN_ON_ONCE(IS_RDONLY(inode));
+
 	if (ext3_journal_current_handle())
 		goto no_write;