tcp: fix tcp_mtup_probe_success vs wrong snd_cwnd

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

tcp: fix tcp_mtup_probe_success vs wrong snd_cwnd

syzbot got a new report [1] finally pointing to a very old bug,
added in initial support for MTU probing.

tcp_mtu_probe() has checks about starting an MTU probe if
tcp_snd_cwnd(tp) >= 11.

But nothing prevents tcp_snd_cwnd(tp) to be reduced later
and before the MTU probe succeeds.

This bug would lead to potential zero-divides.

Debugging added in commit 40570375356c ("tcp: add accessors
to read/set tp->snd_cwnd") has paid off :)

While we are at it, address potential overflows in this code.

[1]
WARNING: CPU: 1 PID: 14132 at include/net/tcp.h:1219 tcp_mtup_probe_success+0x366/0x570 net/ipv4/tcp_input.c:2712
Modules linked in:
CPU: 1 PID: 14132 Comm: syz-executor.2 Not tainted 5.18.0-syzkaller-07857-gbabf0bb978e3 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:tcp_snd_cwnd_set include/net/tcp.h:1219 [inline]
RIP: 0010:tcp_mtup_probe_success+0x366/0x570 net/ipv4/tcp_input.c:2712
Code: 74 08 48 89 ef e8 da 80 17 f9 48 8b 45 00 65 48 ff 80 80 03 00 00 48 83 c4 30 5b 41 5c 41 5d 41 5e 41 5f 5d c3 e8 aa b0 c5 f8 <0f> 0b e9 16 fe ff ff 48 8b 4c 24 08 80 e1 07 38 c1 0f 8c c7 fc ff
RSP: 0018:ffffc900079e70f8 EFLAGS: 00010287
RAX: ffffffff88c0f7f6 RBX: ffff8880756e7a80 RCX: 0000000000040000
RDX: ffffc9000c6c4000 RSI: 0000000000031f9e RDI: 0000000000031f9f
RBP: 0000000000000000 R08: ffffffff88c0f606 R09: ffffc900079e7520
R10: ffffed101011226d R11: 1ffff1101011226c R12: 1ffff1100eadcf50
R13: ffff8880756e72c0 R14: 1ffff1100eadcf89 R15: dffffc0000000000
FS: 00007f643236e700(0000) GS:ffff8880b9b00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f1ab3f1e2a0 CR3: 0000000064fe7000 CR4: 00000000003506e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
tcp_clean_rtx_queue+0x223a/0x2da0 net/ipv4/tcp_input.c:3356
tcp_ack+0x1962/0x3c90 net/ipv4/tcp_input.c:3861
tcp_rcv_established+0x7c8/0x1ac0 net/ipv4/tcp_input.c:5973
tcp_v6_do_rcv+0x57b/0x1210 net/ipv6/tcp_ipv6.c:1476
sk_backlog_rcv include/net/sock.h:1061 [inline]
__release_sock+0x1d8/0x4c0 net/core/sock.c:2849
release_sock+0x5d/0x1c0 net/core/sock.c:3404
sk_stream_wait_memory+0x700/0xdc0 net/core/stream.c:145
tcp_sendmsg_locked+0x111d/0x3fc0 net/ipv4/tcp.c:1410
tcp_sendmsg+0x2c/0x40 net/ipv4/tcp.c:1448
sock_sendmsg_nosec net/socket.c:714 [inline]
sock_sendmsg net/socket.c:734 [inline]
__sys_sendto+0x439/0x5c0 net/socket.c:2119
__do_sys_sendto net/socket.c:2131 [inline]
__se_sys_sendto net/socket.c:2127 [inline]
__x64_sys_sendto+0xda/0xf0 net/socket.c:2127
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x2b/0x70 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x46/0xb0
RIP: 0033:0x7f6431289109
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:00007f643236e168 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
RAX: ffffffffffffffda RBX: 00007f643139c100 RCX: 00007f6431289109
RDX: 00000000d0d0c2ac RSI: 0000000020000080 RDI: 000000000000000a
RBP: 00007f64312e308d R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fff372533af R14: 00007f643236e300 R15: 0000000000022000