seg6: fix seg6 lwtunnel output redirect for L2 reduced encap mode

When SEG6_IPTUN_MODE_L2ENCAP_RED (L2ENCAP_RED) was introduced, the
condition in seg6_build_state() that excludes L2 encap modes from
setting LWTUNNEL_STATE_OUTPUT_REDIRECT was not updated to account for
the new mode.
As a consequence, L2ENCAP_RED routes incorrectly trigger seg6_output()
on the output path, where the packet is silently dropped because
skb_mac_header_was_set() fails on L3 packets.

Extend the check to also exclude L2ENCAP_RED, consistent with L2ENCAP.

Fixes: 13f0296 ("seg6: add support for SRv6 H.L2Encaps.Red behavior")
Cc: stable@vger.kernel.org
Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Reviewed-by: Justin Iurman <justin.iurman@gmail.com>
Link: https://patch.msgid.link/20260418162838.31979-1-andrea.mayer@uniroma2.it
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: skorpion17

net/ipv6/seg6_iptunnel.c

@@ -746,7 +746,8 @@ static int seg6_build_state(struct net *net, struct nlattr *nla,
 	newts->type = LWTUNNEL_ENCAP_SEG6;
 	newts->flags |= LWTUNNEL_STATE_INPUT_REDIRECT;
 
-	if (tuninfo->mode != SEG6_IPTUN_MODE_L2ENCAP)
+	if (tuninfo->mode != SEG6_IPTUN_MODE_L2ENCAP &&
+	    tuninfo->mode != SEG6_IPTUN_MODE_L2ENCAP_RED)
 		newts->flags |= LWTUNNEL_STATE_OUTPUT_REDIRECT;
 
 	newts->headroom = seg6_lwt_headroom(tuninfo);