Bluetooth: SMP: Zeroize raw key data on the stack in smp_e()

smp_e() already clears the AES key in its "struct aes_enckey aes"
on the stack before leaving the function - but the very same
information is also available as raw key data in the tmp[] array,
so this should get cleared, too.

Signed-off-by: Thomas Huth <thuth@redhat.com>