iscsi: Load balancing of iSCSI target is not working

When an iSCSI connection is closed, spdk_iscsi_conn_stop_poller()
is called in spdk_iscsi_conn_destruct() or
spdk_iscsi_conn_check_shutdown().

However, through both paths, g_num_connections[] is decremented
twice:
 (1) in spdk_iscsi_conn_stop_poller()
 (2) in spdk_iscsi_conn_free()
This behavior makes g_num_connections[] negative unexpectedly and
for some cases load balancing does not work and the same CPU is
always selected.

How to fix:
spdk_iscsi_conn_stop_poller() is called in the following functions:
- spdk_iscsi_conn_check_shutdown()
- spdk_iscsi_conn_destruct()
- spdk_iscsi_conn_handle_idle().

[Idea 1]
Remove the code to decrement g_num_connections[] from
spdk_iscsi_conn_free().

[Idea 2]
Remove the code to decrement g_num_connections[] from
spdk_iscsi_conn_stop_poller(). Add the code to decrement
g_num_connections[] to __add_idle_conn().

[Idea 1] is simple because just only one line is deleted.

[Idea 2] may be more symmetric than [Idea 1] but to find the right
place to add the code in __add_idle_conn() is not clear and
difficult.

Hence [Idea 1] is proposed.

[Idea 1] may not be the optimal one and more refactoring may be
done.

Change-Id: I81050f4a0e7b3ddd40896f46ab2eb8de14bbcb3a
Signed-off-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-on: https://review.gerrithub.io/384026
Reviewed-by: Cunyin Chang <cunyin.chang@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Daniel Verkamp <daniel.verkamp@intel.com>

lib/iscsi/conn.c

@@ -678,8 +678,6 @@ _spdk_iscsi_conn_free(void *arg1, void *arg2)
 	pthread_mutex_lock(&g_conns_mutex);
 	spdk_iscsi_remove_conn(conn);
 	pthread_mutex_unlock(&g_conns_mutex);
-
-	__sync_fetch_and_sub(&g_num_connections[spdk_env_get_current_core()], 1);
 }
 
 static void