ovl: Support mounting case-insensitive enabled layers

ovl: Support mounting case-insensitive enabled layers

Drop the restriction for casefold dentries lookup to enable support for
case-insensitive layers in overlayfs.

Support case-insensitive layers with the condition that they should be
uniformly enabled across the stack and (i.e. if the root mount dir has
casefold enabled, so should all the dirs bellow for every layer).

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: André Almeida <andrealmeid@igalia.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>