iser-target: Fix implicit termination of connections

In situations such as bond failover, The new session establishment
implicitly invokes the termination of the old connection.

So, we don't want to wait for the old connection wait_conn to completely
terminate before we accept the new connection and post a login response.

The solution is to deffer the comp_wait completion and the conn_put to
a work so wait_conn will effectively be non-blocking (flush errors are
assumed to come very fast).

We allocate isert_release_wq with WQ_UNBOUND and WQ_UNBOUND_MAX_ACTIVE
to spread the concurrency of release works.

Reported-by: Slava Shwartsman <valyushash@gmail.com>
Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Cc: <stable@vger.kernel.org> # v3.10+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/infiniband/ulp/isert/ib_isert.h

@@ -149,6 +149,7 @@ struct isert_conn {
 	int			conn_fr_pool_size;
 	/* lock to protect fastreg pool */
 	spinlock_t		conn_lock;
+	struct work_struct	release_work;
 #define ISERT_COMP_BATCH_COUNT	8
 	int			conn_comp_batch;
 	struct llist_head	conn_comp_llist;