The reason we have to do parent tracking is combination of (a) the Node
structure has a map to its children (b) FORGET messages don't enumerate
parents of a node, just the node to be forgotten.
Hence to keep the map in sync with the kernel's FS tree, we have to update
the parents of a node on FORGET too.

This passes 90+ iterations of fsstress-loopback.bash .
( https://github.com/rfjakob/gocryptfs/blob/master/stress_tests/fsstress-loopback.bash )

Change-Id: Idea478d25703a59d17086db59c110ac55281176a