irmap: Reverse dev:inode to path mapping

For inotify/fanotifies we cannot always open inodes by a
handle and have to scan directories searching for the inode
path :(

Fortunately, in most of the containers' cases fsnotifies are
put in "typical" places. These are used as hints for scanner.

The best way to go is use openvz's ploop over such filesystems.
Long term solution is to fix NFS to provide opening by handle.

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