ocfs2: Make truncating the extent map an extent_tree_operation.

ocfs2_remove_extent() wants to truncate the extent map if it's
truncating 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>