net/batching: reset Buffers len in WriteBatchTo

In case we land on this branch during a goto retry. Also, protect
Geneve offset from mutation across retries.

Fixes #19927

Signed-off-by: Jordan Whited <jordan@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f