ICMPv6 was never excluded from idle-timeout accounting because is_data()
inspected data[9] for both IPv4 and IPv6. Offset 9 is the Protocol field
in the IPv4 fixed header (RFC 791), but the Next Header field in the IPv6
fixed header sits at offset 6 (RFC 8200 §3). As a result, the idle timer
was reset on every ICMPv6 packet, preventing the server from disconnecting
clients that were sending only control traffic.
Resolves: #724
Signed-off-by: Nikos Mavrogiannopoulos <n.mavrogiannopoulos@gmail.com>
