svcrdma: Use struct xdr_stream to decode ingress transport headers

The logic that checks incoming network headers has to be scrupulous.

De-duplicate: replace open-coded buffer overflow checks with the use
of xdr_stream helpers that are used most everywhere else XDR
decoding is done.

One minor change to the sanity checks: instead of checking the
length of individual segments, cap the length of the whole chunk
to be sure it can fit in the set of pages available in rq_pages.
This should be a better test of whether the server can handle the
chunks in each request.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

include/linux/sunrpc/svc_rdma.h

@@ -132,6 +132,7 @@ struct svc_rdma_recv_ctxt {
 	struct ib_sge		rc_recv_sge;
 	void			*rc_recv_buf;
 	struct xdr_buf		rc_arg;
+	struct xdr_stream	rc_stream;
 	bool			rc_temp;
 	u32			rc_byte_len;
 	unsigned int		rc_page_count;