ext3: Try to avoid starting a transaction in writepage for data=writepage

This does the same as commit 9e80d407
(avoid starting a transaction when no block allocation is needed)
but for data=writeback mode of ext3. We also cleanup the data=ordered
case a bit to stick to coding style...

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>