PR: https://github.com/freenas/freenas/pull/4200

For 11.3 vfs_shadow_copy2 is exposed to end-users to provide an
avenue for reverting to legacy behavior in case of discovery of
show-stopping bug in shadow_copy_zfs. This will be removed in 12.0.
If users enable this and shadow_copy_zfs through the "shadowcopy"
checkbox then previous versions will not work. Raise a validation
error to prevent this configuration.