net: tun: fix tun_napi_alloc_frags()

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

net: tun: fix tun_napi_alloc_frags()

syzbot reported the following crash [1]

Issue came with the blamed commit. Instead of going through
all the iov components, we keep using the first one
and end up with a malformed skb.

[1]

kernel BUG at net/core/skbuff.c:2849 !
Oops: invalid opcode: 0000 [#1] PREEMPT SMP KASAN PTI
CPU: 0 UID: 0 PID: 6230 Comm: syz-executor132 Not tainted 6.13.0-rc1-syzkaller-00407-g96b6fcc0ee41 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 11/25/2024
RIP: 0010:__pskb_pull_tail+0x1568/0x1570 net/core/skbuff.c:2848
Code: 38 c1 0f 8c 32 f1 ff ff 4c 89 f7 e8 92 96 74 f8 e9 25 f1 ff ff e8 e8 ae 09 f8 48 8b 5c 24 08 e9 eb fb ff ff e8 d9 ae 09 f8 90 <0f> 0b 66 0f 1f 44 00 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90
RSP: 0018:ffffc90004cbef30 EFLAGS: 00010293
RAX: ffffffff8995c347 RBX: 00000000fffffff2 RCX: ffff88802cf45a00
RDX: 0000000000000000 RSI: 00000000fffffff2 RDI: 0000000000000000
RBP: ffff88807df0c06a R08: ffffffff8995b084 R09: 1ffff1100fbe185c
R10: dffffc0000000000 R11: ffffed100fbe185d R12: ffff888076e85d50
R13: ffff888076e85c80 R14: ffff888076e85cf4 R15: ffff888076e85c80
FS: 00007f0dca6ea6c0(0000) GS:ffff8880b8600000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f0dca6ead58 CR3: 00000000119da000 CR4: 00000000003526f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
skb_cow_data+0x2da/0xcb0 net/core/skbuff.c:5284
tipc_aead_decrypt net/tipc/crypto.c:894 [inline]
tipc_crypto_rcv+0x402/0x24e0 net/tipc/crypto.c:1844
tipc_rcv+0x57e/0x12a0 net/tipc/node.c:2109
tipc_l2_rcv_msg+0x2bd/0x450 net/tipc/bearer.c:668
__netif_receive_skb_list_ptype net/core/dev.c:5720 [inline]
__netif_receive_skb_list_core+0x8b7/0x980 net/core/dev.c:5762
__netif_receive_skb_list net/core/dev.c:5814 [inline]
netif_receive_skb_list_internal+0xa51/0xe30 net/core/dev.c:5905
gro_normal_list include/net/gro.h:515 [inline]
napi_complete_done+0x2b5/0x870 net/core/dev.c:6256
napi_complete include/linux/netdevice.h:567 [inline]
tun_get_user+0x2ea0/0x4890 drivers/net/tun.c:1982
tun_chr_write_iter+0x10d/0x1f0 drivers/net/tun.c:2057
do_iter_readv_writev+0x600/0x880
vfs_writev+0x376/0xba0 fs/read_write.c:1050
do_writev+0x1b6/0x360 fs/read_write.c:1096
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f