TITLE: KMSAN: uninit-value in packet_set_ring

[  208.131930] ==================================================================
[  208.139343] BUG: KMSAN: uninit-value in packet_set_ring+0x11b8/0x2ff0
[  208.147224] CPU: 0 PID: 12442 Comm: syz-executor0 Tainted: G    B           4.13.0+ #12
[  208.155359] Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
[  208.164705] Call Trace:
[  208.167295]  dump_stack+0x172/0x1c0
[  208.170931]  ? packet_set_ring+0x11b8/0x2ff0
[  208.175334]  kmsan_report+0x145/0x3d0
[  208.179143]  __msan_warning_32+0x65/0xb0
[  208.183202]  packet_set_ring+0x11b8/0x2ff0
[  208.187429]  ? memcmp+0xbc/0x1a0
[  208.190799]  packet_setsockopt+0x1619/0x4e40
[  208.195205]  ? selinux_socket_setsockopt+0x2f1/0x330
[  208.200305]  ? __msan_load_shadow_origin_8+0x5d/0xe0
[  208.205390]  ? packet_ioctl+0x400/0x400
[  208.209340]  SYSC_setsockopt+0x36d/0x4b0
[  208.213383]  SyS_setsockopt+0x76/0xa0
[  208.217163]  entry_SYSCALL_64_fastpath+0x13/0x94
[  208.221889] RIP: 0033:0x4520a9
[  208.225056] RSP: 002b:00007f37efa32c08 EFLAGS: 00000216 ORIG_RAX: 0000000000000036
[  208.232740] RAX: ffffffffffffffda RBX: 00007f37efa33700 RCX: 00000000004520a9
[  208.239987] RDX: 0000000000000005 RSI: 0000000000000107 RDI: 000000000000001e
[  208.247230] RBP: 0000000000a6f870 R08: 000000000000047e R09: 0000000000000000
[  208.254485] R10: 0000000020001000 R11: 0000000000000216 R12: 0000000000000000
[  208.261729] R13: 0000000000a6f7ef R14: 00007f37efa339c0 R15: 000000000000000c
[  208.268977] origin description: ----req_u@packet_setsockopt
[  208.274656] local variable created at:
[  208.278520]  packet_setsockopt+0x133/0x4e40