net/packet: cleanup IPv4 fragment guards

The first packet fragment guard had an additional guard clause that was
incorrectly comparing a length in bytes to a length in octets, and was
also comparing what should have been an entire IPv4 through transport
header length to a subprotocol payload length. The subprotocol header
size guards were otherwise protecting against short transport headers,
as is the conservative non-first fragment minimum offset size. Add an
explicit disallowing of fragmentation for TSMP for the avoidance of
doubt.

Updates #cleanup
Updates #5727

Signed-off-by: James Tucker <james@tailscale.com>

net/packet/header.go

@@ -8,6 +8,7 @@ import (
 	"math"
 )
 
+const igmpHeaderLength = 8
 const tcpHeaderLength = 20
 const sctpHeaderLength = 12