ftrace: Comment that ftrace_func_mapper is freed with free_ftrace_hash()

The structure ftrace_func_mapper only contains a single field and that is
a ftrace_hash. It is used to abstract it out from a normal hash to control
users of how it gets modified.

The freeing of a ftrace_func_mapper structure is:

  free_ftrace_hash(&mapper->hash);

Without context, this looks like a bug. It should be commented that it is
not a bug and it is freed this way.

Cc: Masami Hiramatsu <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Mark Rutland <[email protected]>
Link: https://lore.kernel.org/[email protected]
Signed-off-by: Steven Rostedt (Google) <[email protected]>

Author: rostedt

kernel/trace/ftrace.c

@@ -5208,8 +5208,12 @@ struct ftrace_func_map {
 	void				*data;
 };
 
+/*
+ * Note, ftrace_func_mapper is freed by free_ftrace_hash(&mapper->hash).
+ * The hash field must be the first field.
+ */
 struct ftrace_func_mapper {
-	struct ftrace_hash		hash;
+	struct ftrace_hash		hash;	/* Must be first! */
 };
 
 /**
@@ -5344,6 +5348,7 @@ void free_ftrace_func_mapper(struct ftrace_func_mapper *mapper,
 			}
 		}
 	}
+	/* This also frees the mapper itself */
 	free_ftrace_hash(&mapper->hash);
 }