iscsi-target: Fix ERL=2 ASYNC_EVENT connection pointer bug

This patch fixes a long-standing bug in iscsit_build_conn_drop_async_message()
where during ERL=2 connection recovery, a bogus conn_p pointer could
end up being used to send the ISCSI_OP_ASYNC_EVENT + DROPPING_CONNECTION
notifying the initiator that cmd->logout_cid has failed.

The bug was manifesting itself as an OOPs in iscsit_allocate_cmd() with
a bogus conn_p pointer in iscsit_build_conn_drop_async_message().

Reported-by: Arshad Hussain <[e-mail]>
Reported-by: santosh kulkarni <[e-mail]>
Cc: <[e-mail]> #3.1+
Signed-off-by: Nicholas Bellinger <[e-mail]>

drivers/target/iscsi/iscsi_target.c

···
 {
 	struct iscsi_cmd *cmd;
 	struct iscsi_conn *conn_p;
+	bool found = false;
 
 	/*
 	 * Only send a Asynchronous Message on connections whos network
···
 	list_for_each_entry(conn_p, &conn->sess->sess_conn_list, conn_list) {
 		if (conn_p->conn_state == TARG_CONN_STATE_LOGGED_IN) {
 			iscsit_inc_conn_usage_count(conn_p);
+			found = true;
 			break;
 		}
 	}
 
-	if (!conn_p)
+	if (!found)
 		return;
 
 	cmd = iscsit_allocate_cmd(conn_p, TASK_RUNNING);