stats plugin: Fix /proc/self/io fd leak on plugin deinit

This could have been a problem with doveadm-server processes if they had
service_count>1, because doveadm unloads all plugins between commands.

src/plugins/stats/mail-stats-fill.c

@@ -141,3 +141,9 @@ void mail_stats_fill(struct stats_user *suser, struct mail_stats *stats_r)
 	process_read_io_stats(stats_r);
 	user_trans_stats_get(suser, stats_r);
 }
+
+void mail_stats_fill_global_deinit(void)
+{
+	if (proc_io_fd != -1)
+		i_close_fd(&proc_io_fd);
+}

src/plugins/stats/mail-stats.h

@@ -35,4 +35,6 @@ void mail_stats_fill(struct stats_user *suser, struct mail_stats *mail_stats);
 void mail_stats_add_transaction(struct mail_stats *stats,
 				const struct mailbox_transaction_stats *trans_stats);
 
+void mail_stats_fill_global_deinit(void);
+
 #endif

src/plugins/stats/stats-plugin.c

@@ -469,6 +469,7 @@ void stats_plugin_deinit(void)
 {
 	if (global_stats_conn != NULL)
 		stats_connection_unref(&global_stats_conn);
+	mail_stats_fill_global_deinit();
 	mail_storage_hooks_remove(&stats_mail_storage_hooks);
 	stats_unregister(&mail_stats_item);
 }