Bluetooth: Fix warnings for bt_key_strings and bt_slock_key_strings

After adding proper lockdep annotations for Bluetooth protocols the case
when lockdep is disabled produced two compiler warnings:

net/bluetooth/af_bluetooth.c:60: warning: ‘bt_key_strings’ defined but not used
net/bluetooth/af_bluetooth.c:71: warning: ‘bt_slock_key_strings’ defined but not used

Fix both of them by adding a CONFIG_DEBUG_LOCK_ALLOC conditional around
them and re-arranging the code a little bit.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>