Reorganize private pagecache fields

In order to make the main pagecache system sufficiently generic to be effective
outside of filesystem usage scenarios, we relocate the private_{list,lock} of
struct address_space into struct inode (their only user) as i_list{,_lock}.
Next, private_data is converted back to assoc_mapping, and the host pointer is
made void so as to be agnostic to the pagecache's backing system.