ocfs2: btree support for removal of arbirtrary extents

Add code to the btree paths to support the removal of arbitrary regions
within an existing extent. With proper higher level support this can be used
to "punch holes" in a file. Truncate (a special case of hole punching) could
also be converted to use these methods.
Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>