scsi: iscsi: tracing: Use the new __vstring() helper

Instead of open coding a __dynamic_array() with a fixed length (which
defeats the purpose of the dynamic array in the first place). Use the new
__vstring() helper that will use a va_list and only write enough of the
string into the ring buffer that is needed.

Link: https://lkml.kernel.org/r/20220705224750.715763972@goodmis.org

Cc: Fred Herard <fred.herard@oracle.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

include/trace/events/iscsi.h

@@ -26,12 +26,12 @@ DECLARE_EVENT_CLASS(iscsi_log_msg,
 
 	TP_STRUCT__entry(
 		__string(dname, 	dev_name(dev)		)
-		__dynamic_array(char,	msg, ISCSI_MSG_MAX	)
+		__vstring(msg,		vaf->fmt, vaf->va)
 	),
 
 	TP_fast_assign(
 		__assign_str(dname, dev_name(dev));
-		vsnprintf(__get_str(msg), ISCSI_MSG_MAX, vaf->fmt, *vaf->va);
+		__assign_vstr(msg, vaf->fmt, vaf->va);
 	),
 
 	TP_printk("%s: %s",__get_str(dname),  __get_str(msg)