ext3: Improve truncate error handling

New truncate calling convention allows us to handle errors from
ext3_block_truncate_page(). So reorganize the code so that
ext3_block_truncate_page() is called before we change inode size.

This also removes unnecessary block zeroing from error recovery after failed
buffered writes (zeroing isn't needed because we could have never written
non-zero data to disk). We have to be careful and keep zeroing in direct IO
write error recovery because there we might have already overwritten end of the
last file block.

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