This patch implements counting of used space in inodes in bytes.
New field i_bytes is added and used space modulo 512 is kept in
it (rest is still kept in i_blocks). Functions manipulating both
i_blocks and i_bytes are implemented (inode_add_bytes(), inode_sub_bytes()
and inode_set_bytes()). Filesystems allocating only in whole blocks
can safely ignore i_bytes field and continue using i_blocks...