fifo: Generate unique ID for satellite regfile entry

The plan is to have all file entries have unique ID. Fifo
generates a reg file entry to reuse the whole reg-files
c/r-ing engine (ghosts, open-by-path, etc.) and right now
ID for this entry is the same as for fifo entry.

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