nfscl: Add a LayoutError RPC for NFSv4.2 pNFS mounts

If a pNFS server's DS runs out of disk space, it replies
NFSERR_NOSPC to the client doing writing. For the Linux
client, it then sends a LayoutError RPC to the MDS server to
tell it about the error. This patch adds the same to the
FreeBSD NFSv4.2 pNFS client, to maintain Linux compatible
behaviour, particlularily for non-FreeBSD pNFS servers.

MFC after: 2 weeks