sna: Update the partial buffer allocation size when reusing an old mapping

Whilst the old mapping is guaranteed to be larger than the requested
allocation size, keep track of the actual size allows for better packing
of future buffers. And the code also performs a sanity check that the
buffer is the size we claim it to be...

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

src/sna/kgem.c

@@ -3365,6 +3365,7 @@ struct kgem_bo *kgem_create_buffer(struct kgem *kgem,
 		if (old) {
 			DBG(("%s: reusing cpu map handle=%d for buffer\n",
 			     __FUNCTION__, old->handle));
+			alloc = num_pages(old);
 
 			memcpy(&bo->base, old, sizeof(*old));
 			if (old->rq)