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 TCP_CLOSE
status or has fallback to TCP at mptcp_disconnect() time,
mptcp_do_fastclose() skips setting the `send_fastclose flag` and the later
__mptcp_close_ssk() does not reset anymore the related subflow context.

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

WARNING: CPU: 26 PID: 8996 at net/mptcp/subflow.c:1519 subflow_data_ready (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:subflow_data_ready (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>
tcp_data_ready (net/ipv4/tcp_input.c:5356)
tcp_data_queue (net/ipv4/tcp_input.c:5445)
tcp_rcv_state_process (net/ipv4/tcp_input.c:7165)
tcp_v4_do_rcv (net/ipv4/tcp_ipv4.c:1955)
__release_sock (include/net/sock.h:1158 (discriminator 6) net/core/sock.c:3180 (discriminator 6))
release_sock (net/core/sock.c:3737)
mptcp_sendmsg (net/mptcp/protocol.c:1763 net/mptcp/protocol.c:1857)
inet_sendmsg (net/ipv4/af_inet.c:853 (discriminator 7))
__sys_sendto (net/socket.c:727 (discriminator 15) net/socket.c:742 (discriminator 15) net/socket.c:2244 (discriminator 15))
__x64_sys_sendto (net/socket.c:2247)
do_syscall_64 (arch/x86/entry/syscall_64.c:63 (discriminator 1) arch/x86/entry/syscall_64.c:94 (discriminator 1))
entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)
RIP: 0033:0x7f883326702d

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