CVE-2025-71144: mptcp: ensure context reset on disconnect()

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

mptcp: ensure context reset on disconnect()

After the blamed commit below, if the MPC subflow is already in TCPCLOSE status or has fallback to TCP at mptcpdisconnect() time, mptcpdofastclose() skips setting the sendfastclose flag and the later mptcpclosessk() does not reset anymore the related subflow context.

Any later connection will be created with both the requestmptcp flag and the msk-level fallback status off (it is unconditionally cleared at MPTCP disconnect time), leading to a warning in subflowdataready():

WARNING: CPU: 26 PID: 8996 at net/mptcp/subflow.c:1519 subflowdataready (net/mptcp/subflow.c:1519 (discriminator 13)) Modules linked in: CPU: 26 UID: 0 PID: 8996 Comm: syz.22.39 Not tainted 6.18.0-rc7-05427-g11fc074f6c36 #1 PREEMPT(voluntary) Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011 RIP: 0010:subflowdataready (net/mptcp/subflow.c:1519 (discriminator 13)) Code: 90 0f 0b 90 90 e9 04 fe ff ff e8 b7 1e f5 fe 89 ee bf 07 00 00 00 e8 db 19 f5 fe 83 fd 07 0f 84 35 ff ff ff e8 9d 1e f5 fe 90 <0f> 0b 90 e9 27 ff ff ff e8 8f 1e f5 fe 4c 89 e7 48 89 de e8 14 09 RSP: 0018:ffffc9002646fb30 EFLAGS: 00010293 RAX: 0000000000000000 RBX: ffff88813b218000 RCX: ffffffff825c8435 RDX: ffff8881300b3580 RSI: ffffffff825c8443 RDI: 0000000000000005 RBP: 000000000000000b R08: ffffffff825c8435 R09: 000000000000000b R10: 0000000000000005 R11: 0000000000000007 R12: ffff888131ac0000 R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000 FS: 00007f88330af6c0(0000) GS:ffff888a93dd2000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007f88330aefe8 CR3: 000000010ff59000 CR4: 0000000000350ef0 Call Trace: <TASK> tcpdataready (net/ipv4/tcpinput.c:5356) tcpdataqueue (net/ipv4/tcpinput.c:5445) tcprcvstateprocess (net/ipv4/tcpinput.c:7165) tcpv4dorcv (net/ipv4/tcpipv4.c:1955) releasesock (include/net/sock.h:1158 (discriminator 6) net/core/sock.c:3180 (discriminator 6)) releasesock (net/core/sock.c:3737) mptcpsendmsg (net/mptcp/protocol.c:1763 net/mptcp/protocol.c:1857) inetsendmsg (net/ipv4/afinet.c:853 (discriminator 7)) syssendto (net/socket.c:727 (discriminator 15) net/socket.c:742 (discriminator 15) net/socket.c:2244 (discriminator 15)) x64syssendto (net/socket.c:2247) dosyscall64 (arch/x86/entry/syscall64.c:63 (discriminator 1) arch/x86/entry/syscall64.c:94 (discriminator 1)) entrySYSCALL64afterhwframe (arch/x86/entry/entry64.S:130) RIP: 0033:0x7f883326702d

Address the issue setting an explicit fastclosing flag at fastclose time, and checking such flag after mptcpdofastclose().