ovl: store persistent uuid/fsid with uuid=on

With uuid=on, store a persistent uuid in xattr on the upper dir to
give the overlayfs instance a persistent identifier.

This also makes f_fsid persistent and more reliable for reporting
fid info in fanotify events.

uuid=on is not supported on non-upper overlayfs or with upper fs
that does not support xattrs.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>

Documentation/filesystems/overlayfs.rst

@@ -671,6 +671,9 @@ controlled by the "uuid" mount option, which supports these values:
     UUID of underlying layers is ignored.
 - "on":
     UUID of overlayfs is generated and used to report a unique fsid.
+    UUID is stored in xattr "trusted.overlay.uuid", making overlayfs fsid
+    unique and persistent.  This option requires an overlayfs with upper
+    filesystem that supports xattrs.
 
 
 Volatile mount