NFSD: Trace NFSv4 COMPOUND tags

The Linux NFSv4 client implementation does not use COMPOUND tags,
but the Solaris and MacOS implementations do, and so does pynfs.
Record these eye-catchers in the server's trace buffer to annotate
client requests while troubleshooting.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>

fs/nfsd/nfs4proc.c

@@ -2643,7 +2643,7 @@ nfsd4_proc_compound(struct svc_rqst *rqstp)
 
 	rqstp->rq_lease_breaker = (void **)&cstate->clp;
 
-	trace_nfsd_compound(rqstp, args->client_opcnt);
+	trace_nfsd_compound(rqstp, args->tag, args->taglen, args->client_opcnt);
 	while (!status && resp->opcnt < args->opcnt) {
 		op = &args->ops[resp->opcnt++];
 

fs/nfsd/trace.h

@@ -84,19 +84,26 @@ DEFINE_NFSD_XDR_ERR_EVENT(cant_encode);
 		{ NFSD_MAY_64BIT_COOKIE,	"64BIT_COOKIE" })
 
 TRACE_EVENT(nfsd_compound,
-	TP_PROTO(const struct svc_rqst *rqst,
-		 u32 args_opcnt),
-	TP_ARGS(rqst, args_opcnt),
+	TP_PROTO(
+		const struct svc_rqst *rqst,
+		const char *tag,
+		u32 taglen,
+		u32 opcnt
+	),
+	TP_ARGS(rqst, tag, taglen, opcnt),
 	TP_STRUCT__entry(
 		__field(u32, xid)
-		__field(u32, args_opcnt)
+		__field(u32, opcnt)
+		__string_len(tag, tag, taglen)
 	),
 	TP_fast_assign(
 		__entry->xid = be32_to_cpu(rqst->rq_xid);
-		__entry->args_opcnt = args_opcnt;
+		__entry->opcnt = opcnt;
+		__assign_str_len(tag, tag, taglen);
 	),
-	TP_printk("xid=0x%08x opcnt=%u",
-		__entry->xid, __entry->args_opcnt)
+	TP_printk("xid=0x%08x opcnt=%u tag=%s",
+		__entry->xid, __entry->opcnt, __get_str(tag)
+	)
 )
 
 TRACE_EVENT(nfsd_compound_status,