ovl: Ensure that all layers have the same encoding

ovl: Ensure that all layers have the same encoding

When merging layers from different filesystems with casefold enabled,
all layers should use the same encoding version and have the same flags
to avoid any kind of incompatibility issues.

Also, set the encoding and the encoding flags for the ovl super block as
the same as used by the first valid layer.

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