mptcp: Fix crash due to tcp_tsorted_anchor was initialized before release skb

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

mptcp: Fix crash due to tcp_tsorted_anchor was initialized before release skb

Got crash when doing pressure test of mptcp:

===========================================================================

dst_release: dst:ffffa06ce6e5c058 refcnt:-1

kernel tried to execute NX-protected page - exploit attempt? (uid: 0)

BUG: unable to handle kernel paging request at ffffa06ce6e5c058

PGD 190a01067 P4D 190a01067 PUD 43fffb067 PMD 22e403063 PTE 8000000226e5c063

Oops: 0011 [#1] SMP PTI

CPU: 7 PID: 7823 Comm: kworker/7:0 Kdump: loaded Tainted: G E

Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.2.1 04/01/2014

Call Trace:

? skb_release_head_state+0x68/0x100

? skb_release_all+0xe/0x30

? kfree_skb+0x32/0xa0

? mptcp_sendmsg_frag+0x57e/0x750

? __mptcp_retrans+0x21b/0x3c0

? __switch_to_asm+0x35/0x70

? mptcp_worker+0x25e/0x320

? process_one_work+0x1a7/0x360

? worker_thread+0x30/0x390

? create_worker+0x1a0/0x1a0

? kthread+0x112/0x130

? kthread_flush_work_fn+0x10/0x10

? ret_from_fork+0x35/0x40

===========================================================================

In __mptcp_alloc_tx_skb skb was allocated and skb->tcp_tsorted_anchor will

be initialized, in under memory pressure situation sk_wmem_schedule will

return false and then kfree_skb. In this case skb->_skb_refdst is not null

because_skb_refdst and tcp_tsorted_anchor are stored in the same mem, and

kfree_skb will try to release dst and cause crash.