tunnels: do not assume mac header is set in skb_tunnel_check_pmtu()

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

tunnels: do not assume mac header is set in skb_tunnel_check_pmtu()

Recently added debug in commit f9aefd6b2aa3 ("net: warn if mac header
was not set") caught a bug in skb_tunnel_check_pmtu(), as shown
in this syzbot report [1].

In ndo_start_xmit() paths, there is really no need to use skb->mac_header,
because skb->data is supposed to point at it.

[1] WARNING: CPU: 1 PID: 8604 at include/linux/skbuff.h:2784 skb_mac_header_len include/linux/skbuff.h:2784 [inline]
WARNING: CPU: 1 PID: 8604 at include/linux/skbuff.h:2784 skb_tunnel_check_pmtu+0x5de/0x2f90 net/ipv4/ip_tunnel_core.c:413
Modules linked in:
CPU: 1 PID: 8604 Comm: syz-executor.3 Not tainted 5.19.0-rc2-syzkaller-00443-g8720bd951b8e #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:skb_mac_header_len include/linux/skbuff.h:2784 [inline]
RIP: 0010:skb_tunnel_check_pmtu+0x5de/0x2f90 net/ipv4/ip_tunnel_core.c:413
Code: 00 00 00 00 fc ff df 4c 89 fa 48 c1 ea 03 80 3c 02 00 0f 84 b9 fe ff ff 4c 89 ff e8 7c 0f d7 f9 e9 ac fe ff ff e8 c2 13 8a f9 <0f> 0b e9 28 fc ff ff e8 b6 13 8a f9 48 8b 54 24 70 48 b8 00 00 00
RSP: 0018:ffffc90002e4f520 EFLAGS: 00010212
RAX: 0000000000000324 RBX: ffff88804d5fd500 RCX: ffffc90005b52000
RDX: 0000000000040000 RSI: ffffffff87f05e3e RDI: 0000000000000003
RBP: ffffc90002e4f650 R08: 0000000000000003 R09: 000000000000ffff
R10: 000000000000ffff R11: 0000000000000000 R12: 000000000000ffff
R13: 0000000000000000 R14: 000000000000ffcd R15: 000000000000001f
FS: 00007f3babba9700(0000) GS:ffff8880b9b00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000020000080 CR3: 0000000075319000 CR4: 00000000003506e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
geneve_xmit_skb drivers/net/geneve.c:927 [inline]
geneve_xmit+0xcf8/0x35d0 drivers/net/geneve.c:1107
__netdev_start_xmit include/linux/netdevice.h:4805 [inline]
netdev_start_xmit include/linux/netdevice.h:4819 [inline]
__dev_direct_xmit+0x500/0x730 net/core/dev.c:4309
dev_direct_xmit include/linux/netdevice.h:3007 [inline]
packet_direct_xmit+0x1b8/0x2c0 net/packet/af_packet.c:282
packet_snd net/packet/af_packet.c:3073 [inline]
packet_sendmsg+0x21f4/0x55d0 net/packet/af_packet.c:3104
sock_sendmsg_nosec net/socket.c:714 [inline]
sock_sendmsg+0xcf/0x120 net/socket.c:734
____sys_sendmsg+0x6eb/0x810 net/socket.c:2489
___sys_sendmsg+0xf3/0x170 net/socket.c:2543
__sys_sendmsg net/socket.c:2572 [inline]
__do_sys_sendmsg net/socket.c:2581 [inline]
__se_sys_sendmsg net/socket.c:2579 [inline]
__x64_sys_sendmsg+0x132/0x220 net/socket.c:2579
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x46/0xb0
RIP: 0033:0x7f3baaa89109
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f3babba9168 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00007f3baab9bf60 RCX: 00007f3baaa89109
RDX: 0000000000000000 RSI: 0000000020000a00 RDI: 0000000000000003
RBP: 00007f3baaae305d R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007ffe74f2543f R14: 00007f3babba9300 R15: 0000000000022000
</TASK>