batman-adv: fix panic during interface removal

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

batman-adv: fix panic during interface removal

Reference counting is used to ensure that

batadv_hardif_neigh_node and batadv_hard_iface

are not freed before/during

batadv_v_elp_throughput_metric_update work is

finished.

But there isn't a guarantee that the hard if will

remain associated with a soft interface up until

the work is finished.

This fixes a crash triggered by reboot that looks

like this:

Call trace:

batadv_v_mesh_free+0xd0/0x4dc [batman_adv]

batadv_v_elp_throughput_metric_update+0x1c/0xa4

process_one_work+0x178/0x398

worker_thread+0x2e8/0x4d0

kthread+0xd8/0xdc

ret_from_fork+0x10/0x20

(the batadv_v_mesh_free call is misleading,

and does not actually happen)

I was able to make the issue happen more reliably

by changing hardif_neigh->bat_v.metric_work work

to be delayed work. This allowed me to track down

and confirm the fix.

[[email protected]: prevent entering batadv_v_elp_get_throughput without

soft_iface]