SUNRPC: Fix the svc_deferred_event trace class

Fix a NULL deref crash that occurs when an svc_rqst is deferred
while the sunrpc tracing subsystem is enabled. svc_revisit() sets
dr->xprt to NULL, so it can't be relied upon in the tracepoint to
provide the remote's address.

Unfortunately we can't revert the "svc_deferred_class" hunk in
commit ece200d ("sunrpc: Save remote presentation address in
svc_xprt for trace events") because there is now a specific check
of event format specifiers for unsafe dereferences. The warning
that check emits is:

  event svc_defer_recv has unsafe dereference of argument 1

A "%pISpc" format specifier with a "struct sockaddr *" is indeed
flagged by this check.

Instead, take the brute-force approach used by the svcrdma_qp_error
tracepoint. Convert the dr::addr field into a presentation address
in the TP_fast_assign() arm of the trace event, and store that as
a string. This fix can be backported to -stable kernels.

In the meantime, commit c6ced22 ("tracing: Update print fmt
check to handle new __get_sockaddr() macro") is now in v5.18, so
this wonky fix can be replaced with __sockaddr() and friends
properly during the v5.19 merge window.

Fixes: ece200d ("sunrpc: Save remote presentation address in svc_xprt for trace events")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

include/trace/events/sunrpc.h

@@ -2017,17 +2017,18 @@ DECLARE_EVENT_CLASS(svc_deferred_event,
 	TP_STRUCT__entry(
 		__field(const void *, dr)
 		__field(u32, xid)
-		__string(addr, dr->xprt->xpt_remotebuf)
+		__array(__u8, addr, INET6_ADDRSTRLEN + 10)
 	),
 
 	TP_fast_assign(
 		__entry->dr = dr;
 		__entry->xid = be32_to_cpu(*(__be32 *)(dr->args +
 						       (dr->xprt_hlen>>2)));
-		__assign_str(addr, dr->xprt->xpt_remotebuf);
+		snprintf(__entry->addr, sizeof(__entry->addr) - 1,
+			 "%pISpc", (struct sockaddr *)&dr->addr);
 	),
 
-	TP_printk("addr=%s dr=%p xid=0x%08x", __get_str(addr), __entry->dr,
+	TP_printk("addr=%s dr=%p xid=0x%08x", __entry->addr, __entry->dr,
 		__entry->xid)
 );