blocklayout GETDEVICEINFO fix

[Christoph Hellwig <hch@lst.de>]

The rpc_pipefs pattern used for the GETDEVICEINFO implementation in the
blocklayout driver is fundamentally not thread safe.  Workloads like
dbench manage to trigger concurrent GETDEVICEINFO calls though, so we
need to add a critical section around it.

I also wonder if we should avoid even sending multiple GETDEVICEINFO
a a higher level in the NFS client, as ѕending multiple request just
to cancel out their action before adding them to the cache doesn't
seem very helpful.