target: Make TCM_WRITE_PROTECT failure honor D_SENSE bit

This patch changes transport_lookup_cmd_lun() to obtain
se_lun->lun_ref + se_cmd->se_device rcu_dereference during
TCM_WRITE_PROTECT -> CHECK_CONDITION failure status.

Do this to ensure the active control D_SENSE mode page bit
is being honored.

Reported-by: Sagi Grimberg <sagig@mellanox.com>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>