SUNRPC: Bump the maximum payload size for the server

Increase the maximum server-side RPC payload to 4MB. The default
remains at 1MB.

An API to adjust the operational maximum was added in 2006 by commit
596bbe53eb3a ("[PATCH] knfsd: Allow max size of NFSd payload to be
configured"). To adjust the operational maximum using this API, shut
down the NFS server. Then echo a new value into:

/proc/fs/nfsd/max_block_size

And restart the NFS server.

Reviewed-by: Jeff Layton <[e-mail]>
Reviewed-by: NeilBrown <[e-mail]>
Signed-off-by: Chuck Lever <[e-mail]>

include/linux/sunrpc/svc.h

···
  * Linux limit; someone who cares more about NFS/UDP performance
  * can test a larger number.
  *
- * For TCP transports we have more freedom.  A size of 1MB is
- * chosen to match the client limit.  Other OSes are known to
- * have larger limits, but those numbers are probably beyond
- * the point of diminishing returns.
+ * For non-UDP transports we have more freedom.  A size of 4MB is
+ * chosen to accommodate clients that support larger I/O sizes.
  */
-#define RPCSVC_MAXPAYLOAD	(1*1024*1024u)
-#define RPCSVC_MAXPAYLOAD_TCP	RPCSVC_MAXPAYLOAD
-#define RPCSVC_MAXPAYLOAD_UDP	(32*1024u)
+enum {
+	RPCSVC_MAXPAYLOAD	= 4 * 1024 * 1024,
+	RPCSVC_MAXPAYLOAD_TCP	= RPCSVC_MAXPAYLOAD,
+	RPCSVC_MAXPAYLOAD_UDP	= 32 * 1024,
+};
 
 extern u32 svc_max_payload(const struct svc_rqst *rqstp);