usb: gadget: functionfs: use dma_buf_unmap_attachment_unlocked() helper

Replace the open-coded dma_resv_lock()/dma_resv_unlock() around
dma_buf_unmap_attachment() in ffs_dmabuf_release() with the
dma_buf_unmap_attachment_unlocked() helper.

This aligns FunctionFS DMABUF unmap handling with the standard
DMA-BUF API, avoids duplicating locking logic and eases future
maintenance. No functional change.

Reviewed-by: fanggeng <fanggeng@lixiang.com>
Signed-off-by: Liang Jie <liangjie@lixiang.com>
Link: https://patch.msgid.link/20251114084246.2064845-1-buaajxlj@163.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Liang Jie

drivers/usb/gadget/function/f_fs.c

@@ -1306,9 +1306,7 @@ static void ffs_dmabuf_release(struct kref *ref)
 	struct dma_buf *dmabuf = attach->dmabuf;
 
 	pr_vdebug("FFS DMABUF release\n");
-	dma_resv_lock(dmabuf->resv, NULL);
-	dma_buf_unmap_attachment(attach, priv->sgt, priv->dir);
-	dma_resv_unlock(dmabuf->resv);
+	dma_buf_unmap_attachment_unlocked(attach, priv->sgt, priv->dir);
 
 	dma_buf_detach(attach->dmabuf, attach);
 	dma_buf_put(dmabuf);