Reorganize private pagecache fields

Make the pagecache subsystem sufficiently generic to be practical outside of the
virtual filesystem switch by relocating private_{list,lock} from the
address_space structure into the inode structure (only user) as i_list{,_lock}.
Next, private_data is converted back to assoc_mapping (to be used _only_ for
superset mappings), and the host pointer is made void to be agnostic to the
pagecache's owner.