files: clean up logic about intersection image fd and target fd

open_fdinfo calls move_img_fd, so other functions should not care about it

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