Plumb the outbound injection path to allow passing netstack
PacketBuffers down to the tun Read, where they are decref'd to enable
buffer re-use. This removes one packet alloc & copy, and reduces GC
pressure by pooling outbound injected packets.
Fixes#2741
Signed-off-by: James Tucker <james@tailscale.com>
Primarily this is for f375784d83852b1e3ff20cc9de0648b3c0cf8525 and the
related commits that provide buffer pooling for the endpoint code paths
we use.
Signed-off-by: James Tucker <james@tailscale.com>