Snapshotters: Export the root path

Some of the snapshotters that allow you to change their root location
were already doing this, this just makes all of them follow the same
pattern.

Signed-off-by: Danny Canter <danny@dcantah.dev>

plugins/snapshots/devmapper/plugin/plugin.go

@@ -50,6 +50,7 @@ func init() {
 				config.RootPath = ic.Properties[plugins.PropertyRootDir]
 			}
 
+			ic.Meta.Exports[plugins.SnapshotterRootDir] = config.RootPath
 			return devmapper.NewSnapshotter(ic.Context, config)
 		},
 	})