nbd: upgraded package to upstream

Upgraded sys-block/nbd to version 3.15.3 on amd64, arm, x86

BUG=b:80423674
TEST=emerge-{metis,daisy,oak} sys-block/nbd

Change-Id: Ib69ca957d4bda775a65c0314137e3f8f8d96c7fa
Signed-off-by: Andrew Bresticker <abrestic@waymo.com>
Reviewed-on: https://chromium-review.googlesource.com/1073628
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Andrew Bresticker <abrestic@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
4 files changed