fifo: Sanitize fifo restore

Don't push open cb onto reg_file_into, pass it into regfiles engine
as an argument.

Note: I haven't merged the fifo zdtms yet, thus this patch is untested.

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