xen/virtio: Fix n_pages calculation in xen_grant_dma_map(unmap)_page()

Take page offset into the account when calculating the number of pages
to be granted.

Fixes: d6aca350 ("xen/grant-dma-ops: Add option to restrict memory access under Xen")
Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20221005174823.1800761-2-olekstysh@gmail.comSigned-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/grant-dma-ops.c

@@ -153,7 +153,7 @@ static dma_addr_t xen_grant_dma_map_page(struct device *dev, struct page *page,
 					 unsigned long attrs)
 {
 	struct xen_grant_dma_data *data;
-	unsigned int i, n_pages = PFN_UP(size);
+	unsigned int i, n_pages = PFN_UP(offset + size);
 	grant_ref_t grant;
 	dma_addr_t dma_handle;
 
@@ -185,7 +185,8 @@ static void xen_grant_dma_unmap_page(struct device *dev, dma_addr_t dma_handle,
 				     unsigned long attrs)
 {
 	struct xen_grant_dma_data *data;
-	unsigned int i, n_pages = PFN_UP(size);
+	unsigned long offset = dma_handle & (PAGE_SIZE - 1);
+	unsigned int i, n_pages = PFN_UP(offset + size);
 	grant_ref_t grant;
 
 	if (WARN_ON(dir == DMA_NONE))