eventfd: Add checkpoint/restore support v2

v2:
 - Pass initial counter value to eventfd call
   (can't pass flags here since they are obtained
    with fcntl and must be restored same way or
    restore will fail)
 - Use rst_file_params for flags and owner restore
 - Use eventfd.[ch] instead of eventfs.[ch]
 - Move show funcs to eventfd.c

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>