[BUG] WARNING: refcount bug in gsm_mux_net_start_xmit

Hello,

As part of the kernel research at ASU's SEFCOM
lab, we hit the crash below. Crash reports can be found here:

  https://github.com/farhad-alemi/public_bug_reports/tree/main/199-warning-refcount-bug-in-gsm-mux-net-start-xmit/

  refcount_t: addition on 0; use-after-free.
  WARNING: lib/refcount.c:25 at refcount_warn_saturate+0x9f/0x110
lib/refcount.c:25, CPU#0: kworker/u8:1/14
  Workqueue: ipv6_addrconf addrconf_dad_work
  Call Trace:
   refcount_warn_saturate+0x9f/0x110 lib/refcount.c:25
   __refcount_inc include/linux/refcount.h:366 [inline]
   refcount_inc include/linux/refcount.h:383 [inline]
   kref_get include/linux/kref.h:45 [inline]
   muxnet_get drivers/tty/n_gsm.c:3938 [inline]
   gsm_mux_net_start_xmit+0x3af/0x440 drivers/tty/n_gsm.c:3951
   __netdev_start_xmit include/linux/netdevice.h:5368 [inline]
   dev_hard_start_xmit+0x23b/0x620 net/core/dev.c:3904
   __dev_queue_xmit+0x160b/0x3250 net/core/dev.c:4831
   ip6_finish_output2+0xfd4/0x19b0 net/ipv6/ip6_output.c:136
   mld_sendpack+0x820/0xe00 net/ipv6/mcast.c:1855
   ipv6_mc_dad_complete+0x85/0x470 net/ipv6/mcast.c:2279

Our reproducer.c is available upon request.

Happy to test a patch if that would help.

Regards,