scsi: target: iscsi: Fix use-after-free in iscsit_dec_conn_usage_count()

In iscsit_dec_conn_usage_count(), the function calls complete() while
holding the conn->conn_usage_lock. As soon as complete() is invoked, the
waiter (such as iscsit_close_connection()) may wake up and proceed to free
the iscsit_conn structure.

If the waiter frees the memory before the current thread reaches
spin_unlock_bh(), it results in a KASAN slab-use-after-free as the function
attempts to release a lock within the already-freed connection structure.

Fix this by releasing the spinlock before calling complete().

Signed-off-by: Maurizio Lombardi <[e-mail]>
Reported-by: Zhaojuan Guo <[e-mail]>
Reviewed-by: Mike Christie <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/target/iscsi/iscsi_target_util.c

···
 	spin_lock_bh(&conn->conn_usage_lock);
 	conn->conn_usage_count--;
 
-	if (!conn->conn_usage_count && conn->conn_waiting_on_uc)
+	if (!conn->conn_usage_count && conn->conn_waiting_on_uc) {
+		spin_unlock_bh(&conn->conn_usage_lock);
 		complete(&conn->conn_waiting_on_uc_comp);
+		return;
+	}
 
 	spin_unlock_bh(&conn->conn_usage_lock);
 }