fastrpc: Assert held reservation lock for dma-buf mmapping

When userspace mmaps dma-buf's fd, the dma-buf reservation lock must be
held. Add locking sanity check to the dma-buf mmaping callback to ensure
that the locking assumption won't regress in the future.
Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Acked-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221110201349.351294-7-dmitry.osipenko@collabora.com

drivers/misc/fastrpc.c

@@ -6,6 +6,7 @@
 #include <linux/device.h>
 #include <linux/dma-buf.h>
 #include <linux/dma-mapping.h>
+#include <linux/dma-resv.h>
 #include <linux/idr.h>
 #include <linux/list.h>
 #include <linux/miscdevice.h>
@@ -682,6 +683,8 @@ static int fastrpc_mmap(struct dma_buf *dmabuf,
 	struct fastrpc_buf *buf = dmabuf->priv;
 	size_t size = vma->vm_end - vma->vm_start;
 
+	dma_resv_assert_held(dmabuf->resv);
+
 	return dma_mmap_coherent(buf->dev, vma, buf->virt,
 				 FASTRPC_PHYS(buf->phys), size);
 }