Bluetooth: MGMT: validate LTK enc_size on load

Bluetooth: MGMT: validate LTK enc_size on load

Load Long Term Keys stores the user-provided enc_size and later uses
it to size fixed-size stack operations when replying to LE LTK
requests. An enc_size larger than the 16-byte key buffer can therefore
overflow the reply stack buffer.

Reject oversized enc_size values while validating the management LTK
record so invalid keys never reach the stored key state.

Fixes: 346af67b8d11 ("Bluetooth: Add MGMT handlers for dealing with SMP LTK's")
Reported-by: Keenan Dong <keenanat2000@gmail.com>
Signed-off-by: Keenan Dong <keenanat2000@gmail.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>