Motivation:
When client want's to inform metadata server about errors or statistics
in its interactions with the layout, then LAYOUTERROR and LAYOUTSTATS
operations are called. This information have to be propagated to the pnfs
device manager.

Modification:
Add NFSv41DeviceManager#layoutError and NFSv41DeviceManager#layoutStats
methods. Update corresponding operations to pass requests to device manager.

Result:
Layout usage errors and statistics available to the pnfs device manager.

Acked-by: Paul Millar
Target: master