NFSD: Have legacy NFSD WRITE decoders use xdr_stream_subsegment()

Refactor.

Now that the NFSv2 and NFSv3 XDR decoders have been converted to
use xdr_streams, the WRITE decoder functions can use
xdr_stream_subsegment() to extract the WRITE payload into its own
xdr_buf, just as the NFSv4 WRITE XDR decoder currently does.

That makes it possible to pass the first kvec, pages array + length,
page_base, and total payload length via a single function parameter.

The payload's page_base is not yet assigned or used, but will be in
subsequent patches.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
diff --git a/fs/nfsd/xdr3.h b/fs/nfsd/xdr3.h
index 9330083..712c117 100644
--- a/fs/nfsd/xdr3.h
+++ b/fs/nfsd/xdr3.h
@@ -40,7 +40,7 @@ struct nfsd3_writeargs {
 	__u32			count;
 	int			stable;
 	__u32			len;
-	struct kvec		first;
+	struct xdr_buf		payload;
 };
 
 struct nfsd3_createargs {