Bluetooth: avoid recursive locking in hci_send_to_channel()

Mart reported a deadlock in -RT in the call path:
  hci_send_monitor_ctrl_event() -> hci_send_to_channel()

because both functions acquire the same read lock hci_sk_list.lock. This
is also a mainline issue because the qrwlock implementation is writer
fair (the traditional rwlock implementation is reader biased).

To avoid the deadlock there is now __hci_send_to_channel() which expects
the readlock to be held.

Fixes: 38ceaa00 ("Bluetooth: Add support for sending MGMT commands and events to monitor")
Reported-by: Mart van de Wege <mvdwege@gmail.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>