nfsd: Lower NFSv4.1 callback message size limit

The maximum size of a backchannel message on RPC-over-RDMA depends
on the connection's inline threshold. Today that threshold is
typically 1024 bytes, making the maximum message size 996 bytes.

The Linux server's CREATE_SESSION operation checks that the size
of callback Calls can be as large as 1044 bytes, to accommodate
RPCSEC_GSS. Thus CREATE_SESSION fails if a client advertises the
true message size maximum of 996 bytes.

But the server's backchannel currently does not support RPCSEC_GSS.
The actual maximum size it needs is much smaller. It is safe to
reduce the limit to enable NFSv4.1 on RDMA backchannel operation.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

include/linux/sunrpc/auth.h

@@ -20,11 +20,18 @@
 #include <linux/uidgid.h>
 #include <linux/utsname.h>
 
+/*
+ * Maximum size of AUTH_NONE authentication information, in XDR words.
+ */
+#define NUL_CALLSLACK	(4)
+#define NUL_REPLYSLACK	(2)
+
 /*
  * Size of the nodename buffer. RFC1831 specifies a hard limit of 255 bytes,
  * but Linux hostnames are actually limited to __NEW_UTS_LEN bytes.
  */
 #define UNX_MAXNODENAME	__NEW_UTS_LEN
+#define UNX_CALLSLACK	(21 + XDR_QUADLEN(UNX_MAXNODENAME))
 
 struct rpcsec_gss_info;