usb: gadget: f_subset: Fix unbalanced refcnt in geth_free

geth_alloc() increments the reference count, but geth_free() fails to
decrement it. This prevents the configuration of attributes via configfs
after unlinking the function.

Decrement the reference count in geth_free() to ensure proper cleanup.

Fixes: 02832e5 ("usb: gadget: f_subset: add configfs support")
Cc: stable@vger.kernel.org
Signed-off-by: Kuen-Han Tsai <khtsai@google.com>
Link: https://patch.msgid.link/20260320-usb-net-lifecycle-v1-1-4886b578161b@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Kuen-Han Tsai

drivers/usb/gadget/function/f_subset.c

@@ -6,6 +6,7 @@
  * Copyright (C) 2008 Nokia Corporation
  */
 
+#include <linux/cleanup.h>
 #include <linux/slab.h>
 #include <linux/kernel.h>
 #include <linux/module.h>
@@ -449,8 +450,13 @@ static struct usb_function_instance *geth_alloc_inst(void)
 static void geth_free(struct usb_function *f)
 {
 	struct f_gether *eth;
+	struct f_gether_opts *opts;
+
+	opts = container_of(f->fi, struct f_gether_opts, func_inst);
 
 	eth = func_to_geth(f);
+	scoped_guard(mutex, &opts->lock)
+		opts->refcnt--;
 	kfree(eth);
 }