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/rpc_rdma.h

@@ -58,7 +58,8 @@ enum {
 enum {
 	rpcrdma_fixed_maxsz	= 4,
 	rpcrdma_segment_maxsz	= 4,
-	rpcrdma_readchunk_maxsz	= 2 + rpcrdma_segment_maxsz,
+	rpcrdma_readseg_maxsz	= 1 + rpcrdma_segment_maxsz,
+	rpcrdma_readchunk_maxsz	= 1 + rpcrdma_readseg_maxsz,
 };
 
 /*