Fix issue with grpc message limit (#439)

I'm not sure what's happening, but I get `grpc: received message larger
than max (16777263 vs. 16777216)` error when using imgtool from new
release.

The containerd client should be slicing writes to fit within limit
https://github.com/containerd/containerd/blob/api/v1.10.0/core/content/proxy/content_writer.go#L82
But for some reason that is not happening.

I haven't figured out what's going on, I suggest that we just lower
buffer size in imgtool.

Author: prime000000

img_tool/pkg/load/load.go

@@ -22,7 +22,7 @@ import (
 
 const (
 	defaultWorkers = 4
-	writeBufferSize = 16 * 1024 * 1024
+	writeBufferSize = 8 * 1024 * 1024
 )
 
 type Request struct {