[Bluetooth] Fix deadlock in the L2CAP layer

The Bluetooth L2CAP layer has 2 locks that are used in softirq context,
(one spinlock and one rwlock, where the softirq usage is readlock) but
where not all usages of the lock were _bh safe. The patch below corrects
this.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>