ocfs2_dlmfs: Use the stackglue.

Rather than directly using o2dlm, dlmfs can now use the stackglue.  This
allows it to use userspace cluster stacks and fs/dlm.  This commit
forces o2cb for now.  A latter commit will bump the protocol version and
allow non-o2cb stacks.

This is one big sed, really.  LKM_xxMODE becomes DLM_LOCK_xx.  LKM_flag
becomes DLM_LKF_flag.

We also learn to check that the LVB is valid before reading it.  Any DLM
can lose the contents of the LVB during a complicated recovery.  userdlm
should be checking this.  Now it does.  dlmfs will return 0 from read(2)
if the LVB was invalid.

Signed-off-by: Joel Becker <joel.becker@oracle.com>