iser: remove __packed from struct iser_cm_hdr declaration

`__packed` is not defined previously, and was treated as a varible
declaration.

Signed-off-by: wanghonghao <wanghonghao@bytedance.com>

include/iser-private.h

@@ -158,7 +158,7 @@ struct iser_tx_desc {
 struct iser_cm_hdr {
 	uint8_t      flags;
 	uint8_t      rsvd[3];
-} __packed;
+};
 
 struct iser_pdu {
 	struct iscsi_pdu              iscsi_pdu;