coresight: Clear the connection field properly

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

coresight: Clear the connection field properly

coresight devices track their connections (output connections) and
hold a reference to the fwnode. When a device goes away, we walk through
the devices on the coresight bus and make sure that the references
are dropped. This happens both ways:
a) For all output connections from the device, drop the reference to
the target device via coresight_release_platform_data()

b) Iterate over all the devices on the coresight bus and drop the
reference to fwnode if *this* device is the target of the output
connection, via coresight_remove_conns()->coresight_remove_match().

However, the coresight_remove_match() doesn't clear the fwnode field,
after dropping the reference, this causes use-after-free and
additional refcount drops on the fwnode.

e.g., if we have two devices, A and B, with a connection, A -> B.
If we remove B first, B would clear the reference on B, from A
via coresight_remove_match(). But when A is removed, it still has
a connection with fwnode still pointing to B. Thus it tries to drops
the reference in coresight_release_platform_data(), raising the bells
like :

[ 91.990153] ------------[ cut here ]------------
[ 91.990163] refcount_t: addition on 0; use-after-free.
[ 91.990212] WARNING: CPU: 0 PID: 461 at lib/refcount.c:25 refcount_warn_saturate+0xa0/0x144
[ 91.990260] Modules linked in: coresight_funnel coresight_replicator coresight_etm4x(-)
crct10dif_ce coresight ip_tables x_tables ipv6 [last unloaded: coresight_cpu_debug]
[ 91.990398] CPU: 0 PID: 461 Comm: rmmod Tainted: G W T 5.19.0-rc2+ #53
[ 91.990418] Hardware name: ARM LTD ARM Juno Development Platform/ARM Juno Development Platform, BIOS EDK II Feb 1 2019
[ 91.990434] pstate: 600000c5 (nZCv daIF -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 91.990454] pc : refcount_warn_saturate+0xa0/0x144
[ 91.990476] lr : refcount_warn_saturate+0xa0/0x144
[ 91.990496] sp : ffff80000c843640
[ 91.990509] x29: ffff80000c843640 x28: ffff800009957c28 x27: ffff80000c8439a8
[ 91.990560] x26: ffff00097eff1990 x25: ffff8000092b6ad8 x24: ffff00097eff19a8
[ 91.990610] x23: ffff80000c8439a8 x22: 0000000000000000 x21: ffff80000c8439c2
[ 91.990659] x20: 0000000000000000 x19: ffff00097eff1a10 x18: ffff80000ab99c40
[ 91.990708] x17: 0000000000000000 x16: 0000000000000000 x15: ffff80000abf6fa0
[ 91.990756] x14: 000000000000001d x13: 0a2e656572662d72 x12: 657466612d657375
[ 91.990805] x11: 203b30206e6f206e x10: 6f69746964646120 x9 : ffff8000081aba28
[ 91.990854] x8 : 206e6f206e6f6974 x7 : 69646461203a745f x6 : 746e756f63666572
[ 91.990903] x5 : ffff00097648ec58 x4 : 0000000000000000 x3 : 0000000000000027
[ 91.990952] x2 : 0000000000000000 x1 : 0000000000000000 x0 : ffff00080260ba00
[ 91.991000] Call trace:
[ 91.991012] refcount_warn_saturate+0xa0/0x144
[ 91.991034] kobject_get+0xac/0xb0
[ 91.991055] of_node_get+0x2c/0x40
[ 91.991076] of_fwnode_get+0x40/0x60
[ 91.991094] fwnode_handle_get+0x3c/0x60
[ 91.991116] fwnode_get_nth_parent+0xf4/0x110
[ 91.991137] fwnode_full_name_string+0x48/0xc0
[ 91.991158] device_node_string+0x41c/0x530
[ 91.991178] pointer+0x320/0x3ec
[ 91.991198] vsnprintf+0x23c/0x750
[ 91.991217] vprintk_store+0x104/0x4b0
[ 91.991238] vprintk_emit+0x8c/0x360
[ 91.991257] vprintk_default+0x44/0x50
[ 91.991276] vprintk+0xcc/0xf0
[ 91.991295] _printk+0x68/0x90
[ 91.991315] of_node_release+0x13c/0x14c
[ 91.991334] kobject_put+0x98/0x114
[ 91.991354] of_node_put+0x24/0x34
[ 91.991372] of_fwnode_put+0x40/0x5c
[ 91.991390] fwnode_handle_put+0x38/0x50
[ 91.991411] coresight_release_platform_data+0x74/0xb0 [coresight]
[ 91.991472] coresight_unregister+0x64/0xcc [coresight]
[ 91.991525] etm4_remove_dev+0x64/0x78 [coresight_etm4x]
[ 91.991563] etm4_remove_amba+0x1c/0x2c [coresight_etm4x]
[ 91.991598] amba_remove+0x3c/0x19c
---truncated---