tp->advmss must not be zero, otherwise the kernel maybe crash.

[luqiuwen]

if tp->advmss is zero, the kernel maybe crashed by dividing zero.

Crash Stack. PID: 2817 TASK: ffff95965ce44f10 CPU: 1 COMMAND: "xxxxxxx" #0 [ffff9592f55cb5e0] machine_kexec at ffffffffaa460afa #1 [ffff9592f55cb640] __crash_kexec at ffffffffaa513402 #2 [ffff9592f55cb710] crash_kexec at ffffffffaa5134f0 #3 [ffff9592f55cb728] oops_end at ffffffffaab17738 #4 [ffff9592f55cb750] die at ffffffffaa42e96b #5 [ffff9592f55cb780] do_trap at ffffffffaab16eb0 #6 [ffff9592f55cb7d0] do_divide_error at ffffffffaa42b08e #7 [ffff9592f55cb880] divide_error at ffffffffaab22a5e [exception RIP: tcp_event_data_recv+216] RIP: ffffffffaaa465c8 RSP: ffff9592f55cb938 RFLAGS: 00010246 RAX: 0000000000001000 RBX: ffff9595fbf18000 RCX: 0000000000000000 RDX: 0000000000000000 RSI: ffff9592f5688000 RDI: ffffffffab0fc900 RBP: ffff9592f55cb950 R8: 0000000000000000 R9: ffffffffaaca68c0 R10: 0000000000000014 R11: 0000000000000000 R12: ffff95965b6e0000 R13: 0000000100102bb2 R14: ffff95965b6e0000 R15: ffff95965b6ca04c ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0018 #8 [ffff9592f55cb958] tcp_data_queue at ffffffffaaa49193 Kernel Version: 3.10.0-862.2.3.el7.x86_64