ocfs2: Only build ocfs2/dlm with the o2cb stack module

fs/ocfs2/dlm/ocfs2_dlm.ko and fs/ocfs2/dlm/ocfs2_dlmfs.ko get built if
CONFIG_FS_OCFS2 is specified. This isn't quite how it should happen any more
- the "o2cb" dlm modules should only be built if CONFIG_FS_OCFS2_O2CB is
set, so update the dlm Makefile accordingly.

Signed-off-by: Mark Fasheh <mfasheh@suse.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Joel Becker <joel.becker@oracle.com>