wgengine/filter: don't spam logs on dropped outgoing IPv6 ICMP or IPv4 IGMP

The OS (tries) to send these but we drop them. No need to worry the
user with spam that we're dropping it.

Fixes #402

Signed-off-by: Brad Fitzpatrick <bradfitz@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