fd: Split fdinfo collect routine

In SCM code there will be the need to attach a newly created
FdinfoEntry to a particulat file desc object, then get the
created fdinfo helper object back.

Current code only allows this via two lookup calls -- first
one to attach entry to desc by ID (lookup #1), then get the
fdinfo helper by FD (lookup #2).

Fortunately, the exising code allows simple split that gives
us the optimized routine.

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