target/iscsi: Fix network portal creation race

When creating network portals rapidly, such as when restoring a
configuration, LIO's code to reuse existing portals can return a false
negative if the thread hasn't run yet and set np_thread_state to
ISCSI_NP_THREAD_ACTIVE. This causes an error in the network stack
when attempting to bind to the same address/port.

This patch sets NP_THREAD_ACTIVE before the np is placed on g_np_list,
so even if the thread hasn't run yet, iscsit_get_np will return the
existing np.

Also, convert np_lock -> np_mutex + hold across adding new net portal
to g_np_list to prevent a race where two threads may attempt to create
the same network portal, resulting in one of them failing.

(nab: Add missing mutex_unlocks in iscsit_add_np failure paths)
(DanC: Fix incorrect spin_unlock -> spin_unlock_bh)

Signed-off-by: Andy Grover <agrover@redhat.com>
Cc: <stable@vger.kernel.org> #3.1+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>