radeon/kms: fix dma relocation checking

We were checking the index against the size of the relocation buffer
instead of against the last index. This fix kernel segfault when
userspace submit ill formated command stream/relocation buffer pair.
Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/r600_cs.c

@@ -2563,16 +2563,16 @@ int r600_dma_cs_next_reloc(struct radeon_cs_parser *p,
 	struct radeon_cs_chunk *relocs_chunk;
 	unsigned idx;
 
+	*cs_reloc = NULL;
 	if (p->chunk_relocs_idx == -1) {
 		DRM_ERROR("No relocation chunk !\n");
 		return -EINVAL;
 	}
-	*cs_reloc = NULL;
 	relocs_chunk = &p->chunks[p->chunk_relocs_idx];
 	idx = p->dma_reloc_idx;
-	if (idx >= relocs_chunk->length_dw) {
+	if (idx >= p->nrelocs) {
 		DRM_ERROR("Relocs at %d after relocations chunk end %d !\n",
-			  idx, relocs_chunk->length_dw);
+			  idx, p->nrelocs);
 		return -EINVAL;
 	}
 	*cs_reloc = p->relocs_ptr[idx];