ocfs2: Store the ocfs2_caching_info on ocfs2_extent_tree.

What do we cache?  Metadata blocks.  What are most of our non-inode metadata
blocks?  Extent blocks for our btrees.  struct ocfs2_extent_tree is the
main structure for managing those.  So let's store the associated
ocfs2_caching_info there.

This means that ocfs2_et_root_journal_access() doesn't need struct inode
anymore, and any place that has an et can refer to et->et_ci instead of
INODE_CACHE(inode).

Signed-off-by: Joel Becker <joel.becker@oracle.com>