usb: gadget: functionfs: use dma_buf_unmap_attachment_unlocked() helper

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>