Bluetooth: SMP: Fix invalid access if a buffer cannot be allocated

In certain cases there could be no buffer available which may lead to
NULL being returned by bt_l2cap_create_pdu so instead use
bt_l2cap_create_pdu_timeout to wait with a specific timeout.

Note that this should also ensure the SMP will not be waiting for a
buffer when the remote might have timed out already.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>