s4:dsdb: let dsdb_check_and_update_fl() also operatingSystem[Version]

s4:dsdb: let dsdb_check_and_update_fl() also operatingSystem[Version]

Some clients (e.g. an exchange server) check operatingSystemVersion
in order to check if a domain controller is new enough.

So we better use a value matching the dc functional level.

While we also fixed operatingSystem[Version] at provision time,
we do it also in dsdb_check_and_update_fl() in order to
handle old provisions and systems joined to an existing domain.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>