ext3: Update outdated comment before ext3_ordered_writepage()

The comment is heavily outdated. The recursion into the filesystem isn't
possible because we use GFP_NOFS for our allocations, the issue about
block_write_full_page() dirtying tail page is long resolved as well
(that function doesn't dirty buffers at all), and finally we don't start
a transaction if all blocks are already allocated and mapped.

Signed-off-by: Jan Kara <jack@suse.cz>