files-reg: Add a few helpers to access ghost files remaps from inotify

The remap_put will be needed to defer unlinking of ghost files
if they are referred from inotify system.

The lookup_remap is needed to figure out if the watch
target the inotify has present in ghost files list.

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