Bluetooth: mgmt: fix 'hdev->discovery.uuids' NULL dereference

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

Bluetooth: mgmt: fix 'hdev->discovery.uuids' NULL dereference

'uuid_count' member of struct 'discovery_state' is assigned and read
without any locks, so there is a chance of situation when
uuid_count != 0, but uuids is NULL and there will be NULL pointer
dereference.

Possible race:
'hci_update_passive_scan_sync'
'hci_discovery_filter_clear'
hdev->discovery.uuid_count = 0;
<----------------------preempted----------------------------->
'start_service_discovery'
// Set uuid_count to value != 0
hdev->discovery.uuid_count = uuid_count;
hdev->discovery.uuids = kmemdup(...);
<----------------------preempted----------------------------->
spin_lock(&hdev->discovery.lock);
kfree(hdev->discovery.uuids);
hdev->discovery.uuids = NULL;
spin_unlock(&hdev->discovery.lock);

Now uuids == NULL and uuid_count != 0.
So 'mgmt_device_found' -> 'is_filter_match' -> 'eir_has_uuids' receives
non consistent discovery state, where NULL dereference of uuids happens.

To fix it let's add discovery.lock around every read/write of uuid_count,
uuids pair of struct members. It is also important to assign uuid_count
value only after success kmemdup() allocation in
start_service_discovery(), otherwise uuids is NULL, because kmemdup failed,
but uuid_count is already assigned to non zero value.

The following panic happens:

[ ] ------------[ cut here ]------------
[ ] Unable to handle kernel NULL pointer dereference at virtual
address 0000000000000000
[ ] Internal error: Oops: 0000000096000006 [#1] PREEMPT SMP
[ ] CPU: 0 PID: 15056 Comm: kworker/u9:2
[ ] Workqueue: hci0 hci_rx_work
[ ] pstate: 10400009 (nzcV daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ ] pc : eir_has_uuids+0x2d8/0x590
[ ] lr : is_filter_match+0x258/0x320
...
[ ] Call trace:
[ ] eir_has_uuids+0x2d8/0x590
[ ] is_filter_match+0x258/0x320
[ ] mgmt_device_found+0x5b0/0xafc
[ ] process_adv_report.part.0+0x8c8/0xf14
[ ] hci_le_adv_report_evt+0x338/0x3f0
[ ] hci_le_meta_evt+0x1f0/0x4c8
[ ] hci_event_packet+0x440/0xc9c
[ ] hci_rx_work+0x44c/0xaf8
[ ] process_one_work+0x54c/0x103c
[ ] worker_thread+0x6c4/0x10c4
[ ] kthread+0x274/0x2ec
[ ] ret_from_fork+0x10/0x20
[ ] Code: 14000004 91004021 eb14003f 54000180 (f9400024)
[ ] ---[ end trace 0000000000000000 ]---