ftrace: Reinitialize hash to EMPTY_HASH after freeing

There's several locations that free a ftrace hash pointer but may be
referenced again. Reset them to EMPTY_HASH so that a u-a-f bug doesn't
happen.

Cc: Masami Hiramatsu <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Mathieu Desnoyers <[e-mail]>
Link: https://lore.kernel.org/[e-mail]
Fixes: 0ae6b8ce200d ("ftrace: Fix accounting of subop hashes")
Signed-off-by: Steven Rostedt (Google) <[e-mail]>

kernel/trace/ftrace.c

···
 		return;
 	free_ftrace_hash(ops->func_hash->filter_hash);
 	free_ftrace_hash(ops->func_hash->notrace_hash);
+	ops->func_hash->filter_hash = EMPTY_HASH;
+	ops->func_hash->notrace_hash = EMPTY_HASH;
 }
 EXPORT_SYMBOL_GPL(ftrace_free_filter);
 
···
 				  size_bits);
 		if (ret < 0) {
 			free_ftrace_hash(*filter_hash);
+			*filter_hash = EMPTY_HASH;
 			return ret;
 		}
 	}
···
 				     subops_hash->notrace_hash);
 		if (ret < 0) {
 			free_ftrace_hash(*notrace_hash);
+			*notrace_hash = EMPTY_HASH;
 			return ret;
 		}
 	}