iser: fix crash for sending pdi during reconnecting

Hit the crash stack:
 #0  iser_initialize_headers (iser_pdu=0x7f1a3404ef50, iser_conn=0x0) at iser.c:514
 sahlberg#1  iscsi_iser_send_pdu (iscsi=0x7f1a3406d700, pdu=0x7f1a3404ef50) at iser.c:714
 sahlberg#2  0x000055e3160f0157 in iscsi_scsi_command_async (iscsi=0x7f1a3406d700, iscsi@entry=0x55e317fbcc70,
     lun=lun@entry=1, task=task@entry=0x7f1a34026610, cb=cb@entry=0x55e316044c10 <iscsi_co_generic_cb>,
     d=d@entry=0x7f15feeb7710, private_data=private_data@entry=0x7f15feeb77e0) at iscsi-command.c:282
 sahlberg#3  0x000055e3160f1616 in iscsi_write10_iov_task (iscsi=0x55e317fbcc70, lun=1, lba=lba@entry=10401896,
     data=data@entry=0x0, datalen=4096, blocksize=<optimized out>, wrprotect=0, dpo=0, fua=0, fua_nv=0,
     group_number=0, cb=0x55e316044c10 <iscsi_co_generic_cb>, private_data=0x7f15feeb77e0, iov=0x7f1a34042090,
     niov=1) at iscsi-command.c:1107
 sahlberg#4  0x000055e31604680f in iscsi_co_writev (bs=<optimized out>, sector_num=<optimized out>,
     nb_sectors=<optimized out>, iov=0x7f1a3404e380, flags=<optimized out>) at block/iscsi.c:640
 sahlberg#5  0x000055e31601e89c in bdrv_driver_pwritev (bs=bs@entry=0x55e317fb6570, offset=offset@entry=5325770752,
     bytes=bytes@entry=4096, qiov=qiov@entry=0x7f1a3404e380, qiov_offset=qiov_offset@entry=0, flags=flags@entry=0)
     at block/io.c:1220

The reason is that during async reconnection, before reconnecting
call back function gets woked, we have closed the old connection,
and the new connection is not ready.
At the same time, up layer still sends pdu to the old iscsi context.

In this patch, before reconnecting successfully, just add the pdu to
waitpdu without sending.
Suggested by Bart, do not show iser related log here.

Signed-off-by: zhenwei pi <pizhenwei@bytedance.com>

lib/iser.c

@@ -711,6 +711,13 @@ iscsi_iser_send_pdu(struct iscsi_context *iscsi, struct iscsi_pdu *pdu) {
 	iscsi_pdu_set_expstatsn(pdu, iscsi->statsn + 1);
 	ISCSI_LIST_ADD_END(&iscsi->waitpdu, pdu);
 
+	/* because of async reconnection, before reconnecting successfully,
+	   the argument 'iscsi' is the 'old_iscsi' with a empty connection.
+	 */
+	if (!iser_conn) {
+		return 0;
+	}
+
 	if (iser_initialize_headers(iser_pdu, iser_conn)) {
 		iscsi_set_error(iscsi, "initialize headers Failed\n");
 		return -1;