ocfs2: Support creation of unwritten extents

This can now be trivially supported with re-use of our existing extend code.

ocfs2_allocate_unwritten_extents() takes a start offset and a byte length
and iterates over the inode, adding extents (marked as unwritten) until len
is reached. Existing extents are skipped over.

Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>