ocfs2: Determine an extent tree's max_leaf_clusters in an et_op.

Provide an optional extent_tree_operation to specify the
max_leaf_clusters of an ocfs2_extent_tree.  If not provided, the value
is 0 (unlimited).

Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>