Bluetooth: hci_core: Serialize ACL scheduling with channel deletion

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

Bluetooth: hci_core: Serialize ACL scheduling with channel deletion

hci_chan_sent() selects a channel under RCU but drops the read lock before
accessing chan->conn and returning the channel. The ACL and LE schedulers
then use its packet queue and update its transmit counters without any
protection against channel deletion.

After TX selects a channel and releases RCU, a disconnect command timeout
on the separate request workqueue can run hci_conn_failed() and
l2cap_conn_del(). hci_chan_del() can then unlink the channel, complete
synchronize_rcu(), purge its queue and free it before TX resumes. This
causes use-after-free both in hci_chan_sent() and in its callers.

KASAN reported:

BUG: KASAN: slab-use-after-free in hci_chan_sent+0x892/0x9b0
Workqueue: hci0 hci_tx_work
Call Trace:
hci_chan_sent+0x892/0x9b0
hci_tx_work+0x5e6/0xb70
Allocated by task 91:
hci_chan_create+0xe3/0x350
l2cap_conn_add.part.0+0x12/0xa30
l2cap_chan_connect+0x110d/0x1b60
l2cap_sock_connect+0x310/0x530
Freed by task 99:
hci_chan_del+0x11f/0x170
l2cap_conn_del+0x4f1/0x800
l2cap_connect_cfm+0x88c/0xd30
hci_conn_failed+0x150/0x250
hci_abort_conn_sync+0x3e3/0x800
hci_cmd_sync_run+0x7e/0xc0
hci_abort_conn+0x105/0x1f0
disconnect_sync+0x157/0x290
hci_cmd_sync_work+0x13c/0x290

Hold the existing device mutex across channel selection and transmission
in both schedulers to serialize them with channel teardown. Keep timeout
handling outside the critical sections because hci_link_tx_to() acquires
the same mutex. This also permits the transmit path to sleep, unlike
extending the RCU read-side critical section across packet submission.