inode: Wrap cpt_inode_image into inode_struct structure

I'll use it for tracking pipe/fifo data dumping.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
mm: Drop unused @inode argument from is_dev_zero

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
pipes: Track dumped data

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>