Overlayfs: turn on inodes index feature by default


Linux Kernel Configuration
└─>File systems
└─>Overlayfs: turn on inodes index feature by default
In linux kernel since version 4.2 (release Date: 2015-08-30)  
If this config option is enabled then overlay filesystems will use
the index directory to map lower inodes to upper inodes by default.
In this case it is still possible to turn off index globally with the
"index=off" module option or on a filesystem instance basis with the
"index=off" mount option.

The inodes index feature prevents breaking of lower hardlinks on copy

Note, that the inodes index feature is not backward compatible.
That is, mounting an overlay which has an inodes index on a kernel
that doesn't support this feature will have unexpected results.

If unsure, say N.