ocfs2: Add comment about orphan scanning

 * every slot, queuing a recovery of the slot on the ocfs2_wq thread. This

 * is done to catch any orphans that are left over in orphan directories.

 *

 * It scans all slots, even ones that are in use. It does so to handle the

 * case described below:

 *

 *   Node 1 has an inode it was using. The dentry went away due to memory

 *   pressure.  Node 1 closes the inode, but it's on the free list. The node

 *   has the open lock.

 *   Node 2 unlinks the inode. It grabs the dentry lock to notify others,

 *   but node 1 has no dentry and doesn't get the message. It trylocks the

 *   open lock, sees that another node has a PR, and does nothing.

 *   Later node 2 runs its orphan dir. It igets the inode, trylocks the

 *   open lock, sees the PR still, and does nothing.

 *   Basically, we have to trigger an orphan iput on node 1. The only way

 *   for this to happen is if node 1 runs node 2's orphan dir.

 *

 * ocfs2_queue_orphan_scan gets called every ORPHAN_SCAN_SCHEDULE_TIMEOUT

 * seconds.  It gets an EX lock on os_lockres and checks sequence number

 * stored in LVB. If the sequence number has changed, it means some other