SUNRPC: Fix the svc_deferred_event trace class

In the Linux kernel, the following vulnerability has been resolved:

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 ece200ddd54b ("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 c6ced22997ad ("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.