mptcp: Zero out tcp_skb_cb->dss when reinjecting

In mptcp_write_xmit we expect path_mask to be zero when we are
reinjecting a segment from a subflow.
In 9b5a180b47 (mptcp: rework DSS handling) the path_mask is overriden at
the subflow-level by tcp_skb_cb->dss. This field will effectively be
non-zero when we are removing the skb from the write-queue.

Thus, we must zero it explicitly in __mptcp_reinject_data.

Fixes: 9b5a180b47 (mptcp: rework DSS handling)
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
mptcp: Fix request-sock initialization if TCP_MD5SIG is disabled

In mptcp_v4/6_join_request, we do not set af_specific if TCP_MD5SIG is
disabled in the kernel-config. Thus, af_specific will be garbage and a
call to any of the function-pointers will result in a crash:

[   83.153400] BUG: unable to handle kernel NULL pointer dereference at 0000000000000020
[   83.247329] IP: [<ffffffff815995ee>] tcp_rtx_synack+0x1e/0x50
[   83.316189] PGD 235dfc067 PUD 235447067 PMD 0
[   83.369652] Oops: 0000 [#1] PREEMPT SMP
[   83.416882] Modules linked in:
[   83.453497] CPU: 1 PID: 0 Comm: swapper/1 Not tainted 3.14.0-mptcp #714
[   83.532646] Hardware name: HP ProLiant DL165 G7, BIOS O37 01/26/2011
[   83.608678] task: ffff8802368dc2e0 ti: ffff8802368fe000 task.ti: ffff8802368fe000
[   83.698228] RIP: 0010:[<ffffffff815995ee>]  [<ffffffff815995ee>] tcp_rtx_synack+0x1e/0x50
[   83.796208] RSP: 0018:ffff88023fd038b8  EFLAGS: 00010246
[   83.859753] RAX: 0000000000000000 RBX: ffff88023561e8c0 RCX: ffff88023561e8c0
[   83.945143] RDX: ffff88023fd038b8 RSI: 0000000000000000 RDI: ffff880231689d00
[   84.030526] RBP: ffff88023fd038f8 R08: 0000000000000000 R09: 0000000000000000
[   84.115911] R10: ffff88023fd03938 R11: 0000000000000000 R12: ffff88023561e8c0
[   84.201301] R13: ffff880231689d00 R14: ffff880234a40d00 R15: 0000000000000000
[   84.286686] FS:  00007fb924fb5700(0000) GS:ffff88023fd00000(0000) knlGS:0000000000000000
[   84.383515] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[   84.452261] CR2: 0000000000000020 CR3: 0000000235ed8000 CR4: 00000000000007e0
[   84.537650] Stack:
[   84.561675]  0000000000000000 ffff880200000000 ffff88023fd03938 ffff8802316d5a8e
[   84.650607]  0000000000000001 ffff880234a40d28 0000000000000062 ffff8802316d5a64
[   84.739527]  ffff88023fd03918 ffffffff81588a06 ffff8802316d5a64 0000000000000200
[   84.828454] Call Trace:
[   84.857686]  <IRQ>
[   84.880675]  [<ffffffff81588a06>] inet_rtx_syn_ack+0x16/0x30
[   84.950678]  [<ffffffff815a3f92>] tcp_check_req+0x382/0x670
[   85.017348]  [<ffffffff8165acaa>] ? common_interrupt+0x6a/0x6a
[   85.087138]  [<ffffffff81588c41>] ? inet_csk_search_req+0x1/0xd0
[   85.159008]  [<ffffffff815a1f4f>] tcp_v4_hnd_req+0x6f/0x180
[   85.225680]  [<ffffffff815c9fcb>] ? ipt_do_table+0x20b/0x400
[   85.293389]  [<ffffffff81617bc5>] mptcp_v4_do_rcv+0x1e5/0x2c0
[   85.362140]  [<ffffffff815a29cd>] tcp_v4_do_rcv+0x17d/0x2e0
[   85.428811]  [<ffffffff8161d68b>] mptcp_check_req+0x18b/0x210
[   85.497560]  [<ffffffff815a305f>] tcp_v4_rcv+0x52f/0x7b0

Since 85e733cd0 (tcp: add send_synack method to tcp_request_sock_ops) uses
the callback to send a syn-ack, we must make sure that af_specific is
always initialized.

Fixes: 85e733cd0 (tcp: add send_synack method to tcp_request_sock_ops)
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>