ocfs2: Change metadata caching locks to an operations structure.

ocfs2: Change metadata caching locks to an operations structure.

We don't really want to cart around too many new fields on the
ocfs2_caching_info structure.  So let's wrap all our access of the
parent object in a set of operations.  One pointer on caching_info, and
more flexibility to boot.

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