igc: Fix use-after-free error during reset

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

igc: Fix use-after-free error during reset

Cleans the next descriptor to watch (next_to_watch) when cleaning the
TX ring.

Failure to do so can cause invalid memory accesses. If igc_poll() runs
while the controller is being reset this can lead to the driver try to
free a skb that was already freed.

Log message:

[ 101.525242] refcount_t: underflow; use-after-free.
[ 101.525251] WARNING: CPU: 1 PID: 646 at lib/refcount.c:28 refcount_warn_saturate+0xab/0xf0
[ 101.525259] Modules linked in: sch_etf(E) sch_mqprio(E) rfkill(E) intel_rapl_msr(E) intel_rapl_common(E)
x86_pkg_temp_thermal(E) intel_powerclamp(E) coretemp(E) binfmt_misc(E) kvm_intel(E) kvm(E) irqbypass(E) crc32_pclmul(E)
ghash_clmulni_intel(E) aesni_intel(E) mei_wdt(E) libaes(E) crypto_simd(E) cryptd(E) glue_helper(E) snd_hda_codec_hdmi(E)
rapl(E) intel_cstate(E) snd_hda_intel(E) snd_intel_dspcfg(E) sg(E) soundwire_intel(E) intel_uncore(E) at24(E)
soundwire_generic_allocation(E) iTCO_wdt(E) soundwire_cadence(E) intel_pmc_bxt(E) serio_raw(E) snd_hda_codec(E)
iTCO_vendor_support(E) watchdog(E) snd_hda_core(E) snd_hwdep(E) snd_soc_core(E) snd_compress(E) snd_pcsp(E)
soundwire_bus(E) snd_pcm(E) evdev(E) snd_timer(E) mei_me(E) snd(E) soundcore(E) mei(E) configfs(E) ip_tables(E) x_tables(E)
autofs4(E) ext4(E) crc32c_generic(E) crc16(E) mbcache(E) jbd2(E) sd_mod(E) t10_pi(E) crc_t10dif(E) crct10dif_generic(E)
i915(E) ahci(E) libahci(E) ehci_pci(E) igb(E) xhci_pci(E) ehci_hcd(E)
[ 101.525303] drm_kms_helper(E) dca(E) xhci_hcd(E) libata(E) crct10dif_pclmul(E) cec(E) crct10dif_common(E) tsn(E) igc(E)
e1000e(E) ptp(E) i2c_i801(E) crc32c_intel(E) psmouse(E) i2c_algo_bit(E) i2c_smbus(E) scsi_mod(E) lpc_ich(E) pps_core(E)
usbcore(E) drm(E) button(E) video(E)
[ 101.525318] CPU: 1 PID: 646 Comm: irq/37-enp7s0-T Tainted: G E 5.10.30-rt37-tsn1-rt-ipipe #ipipe
[ 101.525320] Hardware name: SIEMENS AG SIMATIC IPC427D/A5E31233588, BIOS V17.02.09 03/31/2017
[ 101.525322] RIP: 0010:refcount_warn_saturate+0xab/0xf0
[ 101.525325] Code: 05 31 48 44 01 01 e8 f0 c6 42 00 0f 0b c3 80 3d 1f 48 44 01 00 75 90 48 c7 c7 78 a8 f3 a6 c6 05 0f 48
44 01 01 e8 d1 c6 42 00 <0f> 0b c3 80 3d fe 47 44 01 00 0f 85 6d ff ff ff 48 c7 c7 d0 a8 f3
[ 101.525327] RSP: 0018:ffffbdedc0917cb8 EFLAGS: 00010286
[ 101.525329] RAX: 0000000000000000 RBX: ffff98fd6becbf40 RCX: 0000000000000001
[ 101.525330] RDX: 0000000000000001 RSI: ffffffffa6f2700c RDI: 00000000ffffffff
[ 101.525332] RBP: ffff98fd6becc14c R08: ffffffffa7463d00 R09: ffffbdedc0917c50
[ 101.525333] R10: ffffffffa74c3578 R11: 0000000000000034 R12: 00000000ffffff00
[ 101.525335] R13: ffff98fd6b0b1000 R14: 0000000000000039 R15: ffff98fd6be35c40
[ 101.525337] FS: 0000000000000000(0000) GS:ffff98fd6e240000(0000) knlGS:0000000000000000
[ 101.525339] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 101.525341] CR2: 00007f34135a3a70 CR3: 0000000150210003 CR4: 00000000001706e0
[ 101.525343] Call Trace:
[ 101.525346] sock_wfree+0x9c/0xa0
[ 101.525353] unix_destruct_scm+0x7b/0xa0
[ 101.525358] skb_release_head_state+0x40/0x90
[ 101.525362] skb_release_all+0xe/0x30
[ 101.525364] napi_consume_skb+0x57/0x160
[ 101.525367] igc_poll+0xb7/0xc80 [igc]
[ 101.525376] ? sched_clock+0x5/0x10
[ 101.525381] ? sched_clock_cpu+0xe/0x100
[ 101.525385] net_rx_action+0x14c/0x410
[ 101.525388] __do_softirq+0xe9/0x2f4
[ 101.525391] __local_bh_enable_ip+0xe3/0x110
[ 101.525395] ? irq_finalize_oneshot.part.47+0xe0/0xe0
[ 101.525398] irq_forced_thread_fn+0x6a/0x80
[ 101.525401] irq_thread+0xe8/0x180
[ 101.525403] ? wake_threads_waitq+0x30/0x30
[ 101.525406] ? irq_thread_check_affinity+0xd0/0xd0
[ 101.525408] kthread+0x183/0x1a0
[ 101.525412] ? kthread_park+0x80/0x80
[ 101.525415] ret_from_fork+0x22/0x30