SUNRPC: Restructure svc_tcp_recv_record()

Refactor: svc_recvfrom() is going to be converted to read into
bio_vecs in a moment. Unhook the only other caller,
svc_tcp_recv_record(), which just wants to read the 4-byte stream
record marker into a kvec.

While we're in the area, streamline this helper by straight-lining
the hot path, replace dprintk call sites with tracepoints, and
reduce the number of atomic bit operations in this path.

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

include/trace/events/sunrpc.h

@@ -1443,6 +1443,30 @@ TRACE_EVENT(svcsock_new_socket,
 	)
 );
 
+TRACE_EVENT(svcsock_marker,
+	TP_PROTO(
+		const struct svc_xprt *xprt,
+		__be32 marker
+	),
+
+	TP_ARGS(xprt, marker),
+
+	TP_STRUCT__entry(
+		__field(unsigned int, length)
+		__field(bool, last)
+		__string(addr, xprt->xpt_remotebuf)
+	),
+
+	TP_fast_assign(
+		__entry->length = be32_to_cpu(marker) & RPC_FRAGMENT_SIZE_MASK;
+		__entry->last = be32_to_cpu(marker) & RPC_LAST_STREAM_FRAGMENT;
+		__assign_str(addr, xprt->xpt_remotebuf);
+	),
+
+	TP_printk("addr=%s length=%u%s", __get_str(addr),
+		__entry->length, __entry->last ? " (last)" : "")
+);
+
 DECLARE_EVENT_CLASS(svcsock_class,
 	TP_PROTO(
 		const struct svc_xprt *xprt,