xprtrdma: Use smaller buffers for RPC-over-RDMA headers

Commit 949317464b ("xprtrdma: Limit number of RDMA segments in
RPC-over-RDMA headers") capped the number of chunks that may appear
in RPC-over-RDMA headers. The maximum header size can be estimated
and fixed to avoid allocating buffer space that is never used.

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

net/sunrpc/xprtrdma/xprt_rdma.h

@@ -160,7 +160,10 @@ rdmab_to_msg(struct rpcrdma_regbuf *rb)
  * The smallest inline threshold is 1024 bytes, ensuring that
  * at least 750 bytes are available for RPC messages.
  */
-#define RPCRDMA_MAX_HDR_SEGS	(8)
+enum {
+	RPCRDMA_MAX_HDR_SEGS = 8,
+	RPCRDMA_HDRBUF_SIZE = 256,
+};
 
 /*
  * struct rpcrdma_rep -- this structure encapsulates state required to recv