ANDROID: vfs: d_canonical_path for stacked FS

Inotify does not currently know when a filesystem is acting as a wrapper
around another fs. This means that inotify watchers will miss any
modifications to the base file, as well as any made in a separate stacked
fs that points to the same file.
d_canonical_path solves this problem by allowing the fs to map a dentry to
a path in the lower fs. Inotify can use it to find the appropriate place to
watch to be informed of all changes to a file.


Change-Id: Icc7ff8f4798bf907d5c9ce32065a0471db751a26
Test: FileObserverTest and FileObserverTestLegacyPath on Cuttlefish
Bug: 171780975
Signed-off-by: Daniel Rosenberg <drosen@google.com>
[astrachan: Folded 34df4102216e ("ANDROID: fsnotify: Notify lower fs of
 astrachan:           open") into this patch]
Signed-off-by: Alistair Strachan <astrachan@google.com>
Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>
Signed-off-by: Alessio Balsini <balsini@google.com>
Git-commit: f37e05049bf8197901ec042818a64110e59fac41
Git-repo: https://android.googlesource.com/kernel/common/


Signed-off-by: Pradeep P V K <ppvk@codeaurora.org>