Bluetooth: Fix hci_link_tx_to RCU lock usage

Syzbot found a bug "BUG: sleeping function called from invalid context
at kernel/locking/mutex.c:580". It is because hci_link_tx_to holds an
RCU read lock and calls hci_disconnect which would hold a mutex lock
since the commit a13f316e ("Bluetooth: hci_conn: Consolidate code
for aborting connections"). Here's an example call trace:

   __dump_stack lib/dump_stack.c:88 [inline]
   dump_stack_lvl+0xfc/0x174 lib/dump_stack.c:106
   ___might_sleep+0x4a9/0x4d3 kernel/sched/core.c:9663
   __mutex_lock_common kernel/locking/mutex.c:576 [inline]
   __mutex_lock+0xc7/0x6e7 kernel/locking/mutex.c:732
   hci_cmd_sync_queue+0x3a/0x287 net/bluetooth/hci_sync.c:388
   hci_abort_conn+0x2cd/0x2e4 net/bluetooth/hci_conn.c:1812
   hci_disconnect+0x207/0x237 net/bluetooth/hci_conn.c:244
   hci_link_tx_to net/bluetooth/hci_core.c:3254 [inline]
   __check_timeout net/bluetooth/hci_core.c:3419 [inline]
   __check_timeout+0x310/0x361 net/bluetooth/hci_core.c:3399
   hci_sched_le net/bluetooth/hci_core.c:3602 [inline]
   hci_tx_work+0xe8f/0x12d0 net/bluetooth/hci_core.c:3652
   process_one_work+0x75c/0xba1 kernel/workqueue.c:2310
   worker_thread+0x5b2/0x73a kernel/workqueue.c:2457
   kthread+0x2f7/0x30b kernel/kthread.c:319
   ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:298

This patch releases RCU read lock before calling hci_disconnect and
reacquires it afterward to fix the bug.

Fixes: a13f316e ("Bluetooth: hci_conn: Consolidate code for aborting connections")
Signed-off-by: Ying Hsu <yinghsu@chromium.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>