ocfs2: Access and dirty the buffer_head in mark_written.

In __ocfs2_mark_extent_written, when we meet with the situation
of c_split_covers_rec, the old solution just replace the extent
record and forget to access and dirty the buffer_head. This will
cause a problem when the unwritten extent is in an extent block.
So access and dirty it.

Signed-off-by: Tao Ma <tao.ma@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>