tracepoints: fix disable

Impact: fix race

Set the probe array pointer to NULL when the tracepoint is disabled.
The probe array point not being NULL could generate a race condition
where the reader would dereference a freed pointer.
Signed-off-by: NMathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/tracepoint.c

@@ -262,6 +262,7 @@ static void set_tracepoint(struct tracepoint_entry **entry,
 static void disable_tracepoint(struct tracepoint *elem)
 {
 	elem->state = 0;
+	rcu_assign_pointer(elem->funcs, NULL);
 }
 
 /**