files: Close old service fd in clone_service_fd()

Next patches will make service_fd_base not contant.
It will be "floating" and change from task to task.
This patch makes preparation for that: it closes
old service fd after it's duplicated.

Currently the code is unused as in case of
!(rsti(me)->clone_flags & CLONE_FILES), the child
has the same id as its parent, and the duplication
just does not occur.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>