drm/xe/userptr: Fix an incorrect assert

The assert incorrectly checks the total length processed which
can in fact be greater than the number of pages. Fix.

Fixes: 0a98219bcc96 ("drm/xe/hmm: Don't dereference struct page pointers without notifier lock")
Cc: Matthew Auld <[e-mail]>
Cc: Matthew Brost <[e-mail]>
Signed-off-by: Thomas Hellström <[e-mail]>
Reviewed-by: Matthew Auld <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]
(cherry picked from commit 70e5043ba85eae199b232e39921abd706b5c1fa4)
Signed-off-by: Rodrigo Vivi <[e-mail]>

drivers/gpu/drm/xe/xe_hmm.c

···
 		i += size;
 
 		if (unlikely(j == st->nents - 1)) {
+			xe_assert(xe, i >= npages);
 			if (i > npages)
 				size -= (i - npages);
+
 			sg_mark_end(sgl);
+		} else {
+			xe_assert(xe, i < npages);
 		}
+
 		sg_set_page(sgl, page, size << PAGE_SHIFT, 0);
 	}
-	xe_assert(xe, i == npages);
 
 	return dma_map_sgtable(dev, st, write ? DMA_BIDIRECTIONAL : DMA_TO_DEVICE,
 			       DMA_ATTR_SKIP_CPU_SYNC | DMA_ATTR_NO_KERNEL_MAPPING);