CVE-2025-71098: ip6_gre: make ip6gre_header() robust

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

ip6gre: make ip6greheader() robust

Over the years, syzbot found many ways to crash the kernel in ip6greheader() [1].

This involves team or bonding drivers ability to dynamically change their dev->neededheadroom and/or dev->hardheaderlen

In this particular crash mldnewpack() allocated an skb with a too small reserve/headroom, and by the time mldsendpack() was called, syzbot managed to attach an ip6gre device.

[1] skbuff: skbunderpanic: text:ffffffff8a1d69a8 len:136 put:40 head:ffff888059bc7000 data:ffff888059bc6fe8 tail:0x70 end:0x6c0 dev:team0 ------------[ cut here ]------------ kernel BUG at net/core/skbuff.c:213 ! <TASK> skbunderpanic net/core/skbuff.c:223 [inline] skbpush+0xc3/0xe0 net/core/skbuff.c:2641 ip6greheader+0xc8/0x790 net/ipv6/ip6gre.c:1371 devhardheader include/linux/netdevice.h:3436 [inline] neighconnectedoutput+0x286/0x460 net/core/neighbour.c:1618 neighoutput include/net/neighbour.h:556 [inline] ip6finishoutput2+0xfb3/0x1480 net/ipv6/ip6output.c:136 ip6finishoutput net/ipv6/ip6output.c:-1 [inline] ip6finishoutput+0x234/0x7d0 net/ipv6/ip6output.c:220 NFHOOKCOND include/linux/netfilter.h:307 [inline] ip6output+0x340/0x550 net/ipv6/ip6output.c:247 NFHOOK+0x9e/0x380 include/linux/netfilter.h:318 mldsendpack+0x8d4/0xe60 net/ipv6/mcast.c:1855 mldsendcr net/ipv6/mcast.c:2154 [inline] mldifcwork+0x83e/0xd60 net/ipv6/mcast.c:2693