ipv6: sr: use skb_get_hash_net() in seg6_make_flowlabel()

In the Linux kernel, the following vulnerability has been resolved:

ipv6: sr: use skb_get_hash_net() in seg6_make_flowlabel()

Since commit d58e468b1112 ("flow_dissector: implements flow dissector
BPF hook") __skb_flow_dissect() needs a net pointer, either from
skb->dev, skb->sk, or since commit 3cbf4ffba5ee ("net: plumb network
namespace into __skb_flow_dissect") a caller provided pointer.

syzbot was able to reach seg6_make_flowlabel() with an skb having
neither skb->dev nor skb->sk set: a TIPC UDP bearer sends a discovery
message through an IPv4 route using seg6 encap, while
net.ipv6.seg6_flowlabel is set to 1.

seg6_make_flowlabel() already has a net pointer, use skb_get_hash_net().

WARNING: net/core/flow_dissector.c:1131 at __skb_flow_dissect+0x910/0x5368 net/core/flow_dissector.c:1126, CPU#0: syz.0.17/4930
Call trace:
__skb_flow_dissect+0x910/0x5368 net/core/flow_dissector.c:1126 (P)
__skb_get_hash_net+0xe0/0x29c net/core/flow_dissector.c:1903
skb_get_hash include/linux/skbuff.h:1663 [inline]
seg6_make_flowlabel+0xcc/0x1ec net/ipv6/seg6_iptunnel.c:132
__seg6_do_srh_encap+0x320/0xbf4 net/ipv6/seg6_iptunnel.c:161
seg6_do_srh+0x4b4/0xa44 net/ipv6/seg6_iptunnel.c:431
seg6_output_core+0x164/0x688 net/ipv6/seg6_iptunnel.c:681
seg6_output+0x44/0x1ac net/ipv6/seg6_iptunnel.c:744
lwtunnel_output+0x3d0/0x664 net/core/lwtunnel.c:356
dst_output include/net/dst.h:470 [inline]
ip_local_out+0x110/0x148 net/ipv4/ip_output.c:131
iptunnel_xmit+0x50c/0xd38 net/ipv4/ip_tunnel_core.c:97
udp_tunnel_xmit_skb+0x220/0x348 net/ipv4/udp_tunnel_core.c:187
tipc_udp_xmit+0x75c/0x9c0 net/tipc/udp_media.c:202
tipc_udp_send_msg+0x214/0x374 net/tipc/udp_media.c:274
tipc_bearer_xmit_skb+0x260/0x3b0 net/tipc/bearer.c:576
tipc_enable_bearer net/tipc/bearer.c:366 [inline]
__tipc_nl_bearer_enable+0xc90/0xfb0 net/tipc/bearer.c:1048
tipc_nl_bearer_enable+0x2c/0x48 net/tipc/bearer.c:1057
genl_family_rcv_msg_doit+0x1e4/0x2d4 net/netlink/genetlink.c:1114