ocfs2: Wrap ocfs2_extent_contig in ocfs2_extent_tree.

Add a new operation eo_ocfs2_extent_contig int the extent tree's
operations vector. So that with the new refcount tree, We want
this so that refcount trees can always return CONTIG_NONE and
prevent extent merging.

Signed-off-by: Tao Ma <tao.ma@oracle.com>