No need to make callers specify the redundant IP version or
TTL/HopLimit or EthernetType in the common case. The mkPacket helper
can set those when unset.
And use the mkIPLayer in another place, simplifying some code.
And rename mkPacketErr to just mkPacket, then move mkPacket to
test-only code, as mustPacket.
Updates #13038
Change-Id: Ic216e44dda760c69ab9bfc509370040874a47d30
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
3d9e3a17fa