parallels: create parallels_set_bat_entry_helper() to assign BAT value

parallels: create parallels_set_bat_entry_helper() to assign BAT value

This helper will be reused in next patches during parallels_co_check
rework to simplify its code.

Signed-off-by: Alexander Ivanov <alexander.ivanov@virtuozzo.com>
Reviewed-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-Id: <20230424093147.197643-5-alexander.ivanov@virtuozzo.com>
Reviewed-by: Hanna Czenczek <hreitz@redhat.com>
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>