Now, nodeId and lookupCount are exclusively used for kernel
communication, and a nodeId may change during the lifetime of the
Inode if the same node is forgotten and looked up again.

treeLock.

FsNode has no Inode.

If not (in case of hardlink), increase the lookupCount for the inode.

To use hard-links, the FileSystem should return consistent
FileInfo.Ino data from the GetAttr call.

* Always take R/W lock

* If the child exists, do addLookupCount() before GetAttr, so the
  child can't be cleaned up during the unprotected GetAttr() call.

errors.

* Introduce Inode.addLookupCount(), checking for underflow. 

* Increase lookupCount for mounts only once

* If FsNode.Link() returns existing node, increase its lookup count.

* Root node should have lookupCount = 1 at start.

* If we get a Lookup for an existing node, increase its lookup count.