NFSD: Return nfserr_serverfault if splice_ok but buf->pages have data

This was discussed with Chuck as part of this patch set. Returning
nfserr_resource was decided to not be the best error message here, and
he suggested changing to nfserr_serverfault instead.

Signed-off-by: Anna Schumaker <[email protected]>
Link: https://lore.kernel.org/linux-nfs/[email protected]/T/#t
Signed-off-by: Chuck Lever <[email protected]>

fs/nfsd/nfs4xdr.c

@@ -3991,7 +3991,7 @@ nfsd4_encode_read(struct nfsd4_compoundres *resp, __be32 nfserr,
 	}
 	if (resp->xdr->buf->page_len && splice_ok) {
 		WARN_ON_ONCE(1);
-		return nfserr_resource;
+		return nfserr_serverfault;
 	}
 	xdr_commit_encode(xdr);