fsnotify: robust support for detecting changes to k8s config objects

fsnotify: robust support for detecting changes to k8s config objects

Kubernetes uses a fairly crazy scheme for files mounted from config maps and in-memory CSI drivers:

  /mnt/file     -> ..data/file
  /mnt/..data   -> ..TIME
  /mnt/..TIME/file (real file)

New versions of the config state are published atomically by doing

  symlink(/mnt/..NEWTIME, /mnt/..data_new)
  rename(/mnt/..data_new, /mnt/..data)

To detect changes to the contents of /mnt/file using inotify instead of polling file contents, we need to detect when /mnt/..data is overwritten.

This requires complex symlink detection and resolution for not only the file but any of the directories in its path, and tracking of final absolute path and inode number to detect when the file was swapped with a different one.

This commit adds that support to fsnotify, along with lots of documentation and tests.