tcp: tcp_make_synack() can be called from process context

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

tcp: tcp_make_synack() can be called from process context

tcp_rtx_synack() now could be called in process context as explained in

0a375c822497 ("tcp: tcp_rtx_synack() can be called from process

context").

tcp_rtx_synack() might call tcp_make_synack(), which will touch per-CPU

variables with preemption enabled. This causes the following BUG:

BUG: using __this_cpu_add() in preemptible [00000000] code: ThriftIO1/5464

caller is tcp_make_synack+0x841/0xac0

Call Trace:

<TASK>

dump_stack_lvl+0x10d/0x1a0

check_preemption_disabled+0x104/0x110

tcp_make_synack+0x841/0xac0

tcp_v6_send_synack+0x5c/0x450

tcp_rtx_synack+0xeb/0x1f0

inet_rtx_syn_ack+0x34/0x60

tcp_check_req+0x3af/0x9e0

tcp_rcv_state_process+0x59b/0x2030

tcp_v6_do_rcv+0x5f5/0x700

release_sock+0x3a/0xf0

tcp_sendmsg+0x33/0x40

____sys_sendmsg+0x2f2/0x490

__sys_sendmsg+0x184/0x230

do_syscall_64+0x3d/0x90

Avoid calling __TCP_INC_STATS() with will touch per-cpu variables. Use

TCP_INC_STATS() which is safe to be called from context switch.