ocfs2: Make extent map insertion an extent_tree_operation.

ocfs2_insert_extent() wants to insert a record into the extent map if
it's an inode data extent.  But since many btrees can call that
function, let's make it an op on ocfs2_extent_tree.  Other tree types
can leave it empty.

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