Bluetooth: Fix potential use-after-free when clear keys

Similar to commit c5d2b6fa ("Bluetooth: Fix use-after-free in
hci_remove_ltk/hci_remove_irk"). We can not access k after kfree_rcu()
call.

Fixes: d7d41682

 ("Bluetooth: Fix Suspicious RCU usage warnings")
Signed-off-by: Min Li <lm0963hack@gmail.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>