hv_netvsc: Fix netvsc_start_xmit's return type

netvsc_start_xmit is used as a callback function for the ndo_start_xmit
function pointer. ndo_start_xmit's return type is netdev_tx_t but
netvsc_start_xmit's return type is int.

This causes a failure with Control Flow Integrity (CFI), which requires
function pointer prototypes and callback function definitions to match
exactly. When CFI is in enforcing, the kernel panics. When booting a
CFI kernel with WSL 2, the VM is immediately terminated because of this.

The splat when CONFIG_CFI_PERMISSIVE is used:

[    5.916765] CFI failure (target: netvsc_start_xmit+0x0/0x10):
[    5.916771] WARNING: CPU: 8 PID: 0 at kernel/cfi.c:29 __cfi_check_fail+0x2e/0x40
[    5.916772] Modules linked in:
[    5.916774] CPU: 8 PID: 0 Comm: swapper/8 Not tainted 5.7.0-rc3-next-20200424-microsoft-cbl-00001-ged4eb37d2c69-dirty #1
[    5.916776] RIP: 0010:__cfi_check_fail+0x2e/0x40
[    5.916777] Code: 48 c7 c7 70 98 63 a9 48 c7 c6 11 db 47 a9 e8 69 55 59 00 85 c0 75 02 5b c3 48 c7 c7 73 c6 43 a9 48 89 de 31 c0 e8 12 2d f0 ff <0f> 0b 5b c3 00 00 cc cc 00 00 cc cc 00 00 cc cc 00 00 85 f6 74 25
[    5.916778] RSP: 0018:ffffa803c0260b78 EFLAGS: 00010246
[    5.916779] RAX: 712a1af25779e900 RBX: ffffffffa8cf7950 RCX: ffffffffa962cf08
[    5.916779] RDX: ffffffffa9c36b60 RSI: 0000000000000082 RDI: ffffffffa9c36b5c
[    5.916780] RBP: ffff8ffc4779c2c0 R08: 0000000000000001 R09: ffffffffa9c3c300
[    5.916781] R10: 0000000000000151 R11: ffffffffa9c36b60 R12: ffff8ffe39084000
[    5.916782] R13: ffffffffa8cf7950 R14: ffffffffa8d12cb0 R15: ffff8ffe39320140
[    5.916784] FS:  0000000000000000(0000) GS:ffff8ffe3bc00000(0000) knlGS:0000000000000000
[    5.916785] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    5.916786] CR2: 00007ffef5749408 CR3: 00000002f4f5e000 CR4: 0000000000340ea0
[    5.916787] Call Trace:
[    5.916788]  <IRQ>
[    5.916790]  __cfi_check+0x3ab58/0x450e0
[    5.916793]  ? dev_hard_start_xmit+0x11f/0x160
[    5.916795]  ? sch_direct_xmit+0xf2/0x230
[    5.916796]  ? __dev_queue_xmit.llvm.11471227737707190958+0x69d/0x8e0
[    5.916797]  ? neigh_resolve_output+0xdf/0x220
[    5.916799]  ? neigh_connected_output.cfi_jt+0x8/0x8
[    5.916801]  ? ip6_finish_output2+0x398/0x4c0
[    5.916803]  ? nf_nat_ipv6_out+0x10/0xa0
[    5.916804]  ? nf_hook_slow+0x84/0x100
[    5.916807]  ? ip6_input_finish+0x8/0x8
[    5.916807]  ? ip6_output+0x6f/0x110
[    5.916808]  ? __ip6_local_out.cfi_jt+0x8/0x8
[    5.916810]  ? mld_sendpack+0x28e/0x330
[    5.916811]  ? ip_rt_bug+0x8/0x8
[    5.916813]  ? mld_ifc_timer_expire+0x2db/0x400
[    5.916814]  ? neigh_proxy_process+0x8/0x8
[    5.916816]  ? call_timer_fn+0x3d/0xd0
[    5.916817]  ? __run_timers+0x2a9/0x300
[    5.916819]  ? rcu_core_si+0x8/0x8
[    5.916820]  ? run_timer_softirq+0x14/0x30
[    5.916821]  ? __do_softirq+0x154/0x262
[    5.916822]  ? native_x2apic_icr_write+0x8/0x8
[    5.916824]  ? irq_exit+0xba/0xc0
[    5.916825]  ? hv_stimer0_vector_handler+0x99/0xe0
[    5.916826]  ? hv_stimer0_callback_vector+0xf/0x20
[    5.916826]  </IRQ>
[    5.916828]  ? hv_stimer_global_cleanup.cfi_jt+0x8/0x8
[    5.916829]  ? raw_setsockopt+0x8/0x8
[    5.916830]  ? default_idle+0xe/0x10
[    5.916832]  ? do_idle.llvm.10446269078108580492+0xb7/0x130
[    5.916833]  ? raw_setsockopt+0x8/0x8
[    5.916833]  ? cpu_startup_entry+0x15/0x20
[    5.916835]  ? cpu_hotplug_enable.cfi_jt+0x8/0x8
[    5.916836]  ? start_secondary+0x188/0x190
[    5.916837]  ? secondary_startup_64+0xa5/0xb0
[    5.916838] ---[ end trace f2683fa869597ba5 ]---

Avoid this by using the right return type for netvsc_start_xmit.

Fixes: fceaf24a ("Staging: hv: add the Hyper-V virtual network driver")
Link: https://github.com/ClangBuiltLinux/linux/issues/1009


Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>