iscsi-target: Fix iscsit_get_tpg_from_np tpg_state bug

This patch fixes a bug in iscsit_get_tpg_from_np() where the
tpg->tpg_state sanity check was looking for TPG_STATE_FREE,
instead of != TPG_STATE_ACTIVE.

The latter is expected during a normal TPG shutdown once the
tpg_state goes into TPG_STATE_INACTIVE in order to reject any
new incoming login attempts.

Cc: <stable@vger.kernel.org> #3.10+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>