ocfs2: Split o2cb code from generic stack functions.

Split off the o2cb-specific funtionality from the generic stack glue
calls.  This is a precurser to wrapping the o2cb functionality in an
operations vector.

Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>