dma-buf: Fix pin callback comment

The pin callback does not necessarily have to move the memory to system
memory, remove the sentence from the comment.

Link: https://lore.kernel.org/r/20211012120903.96933-2-galpress@amazon.comSigned-off-by: Gal Pressman <galpress@amazon.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

include/linux/dma-buf.h

@@ -86,8 +86,8 @@ struct dma_buf_ops {
 	 * @pin:
 	 *
 	 * This is called by dma_buf_pin() and lets the exporter know that the
-	 * DMA-buf can't be moved any more. The exporter should pin the buffer
-	 * into system memory to make sure it is generally accessible by other
+	 * DMA-buf can't be moved any more. Ideally, the exporter should
+	 * pin the buffer so that it is generally accessible by all
 	 * devices.
 	 *
 	 * This is called with the &dmabuf.resv object locked and is mutual