Ocfs2/move_extents: defrag a range of extent.

It's a relatively complete function to accomplish defragmentation for entire
or partial extent, one journal handle was kept during the operation, it was
logically doing one more thing than ocfs2_move_extent() acutally, yes, it's
claiming the new clusters itself;-)

Signed-off-by: Tristan Ye <tristan.ye@oracle.com>