tracing: Use a copy of the va_list for __assign_vstr()

If an instance of tracing enables the same trace event as another
instance, or the top level instance, or even perf, then the va_list passed
into some tracepoints can be used more than once.

As va_list can only be traversed once, this can cause issues:

 # cat /sys/kernel/tracing/instances/qla2xxx/trace
             cat-56106   [012] ..... 2419873.470098: ql_dbg_log: qla2xxx [0000:05:00.0]-1054:14:  Entered (null).
             cat-56106   [012] ..... 2419873.470101: ql_dbg_log: qla2xxx [0000:05:00.0]-1000:14:  Entered ×+<96>²Ü<98>^H.
             cat-56106   [012] ..... 2419873.470102: ql_dbg_log: qla2xxx [0000:05:00.0]-1006:14:  Prepare to issue mbox cmd=0xde589000.

 # cat /sys/kernel/tracing/trace
             cat-56106   [012] ..... 2419873.470097: ql_dbg_log: qla2xxx [0000:05:00.0]-1054:14:  Entered qla2x00_get_firmware_state.
             cat-56106   [012] ..... 2419873.470100: ql_dbg_log: qla2xxx [0000:05:00.0]-1000:14:  Entered qla2x00_mailbox_command.
             cat-56106   [012] ..... 2419873.470102: ql_dbg_log: qla2xxx [0000:05:00.0]-1006:14:  Prepare to issue mbox cmd=0x69.

The instance version is corrupted because the top level instance iterated
the va_list first.

Use va_copy() in the __assign_vstr() macro to make sure that each trace
event for each use case gets a fresh va_list.

Link: https://lore.kernel.org/all/259d53a5-958e-6508-4e45-74dba2821242@marvell.com/
Link: https://lkml.kernel.org/r/20220719182004.21daa83e@gandalf.local.home

Fixes: 0563231f ("tracing/events: Add __vstring() and __assign_vstr() helper macros")
Reported-by: Arun Easi <aeasi@marvell.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

include/trace/stages/stage6_event_callback.h

@@ -40,7 +40,12 @@
 
 #undef __assign_vstr
 #define __assign_vstr(dst, fmt, va)					\
-	vsnprintf(__get_str(dst), TRACE_EVENT_STR_MAX, fmt, *(va))
+	do {								\
+		va_list __cp_va;					\
+		va_copy(__cp_va, *(va));				\
+		vsnprintf(__get_str(dst), TRACE_EVENT_STR_MAX, fmt, __cp_va); \
+		va_end(__cp_va);					\
+	} while (0)
 
 #undef __bitmask
 #define __bitmask(item, nr_bits) __dynamic_array(unsigned long, item, -1)