iio: buffer: use dma_buf_unmap_attachment_unlocked() helper

iio: buffer: use dma_buf_unmap_attachment_unlocked() helper

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

This aligns 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>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>