s390/qeth: add __printf format attribute to qeth_dbf_longtext

Allow the compiler to recognize and check format strings and parameters.

As reported with allmodconfig and W=1:

drivers/s390/net/qeth_core_main.c: In function ‘qeth_dbf_longtext’:
drivers/s390/net/qeth_core_main.c:6190:9: error: function ‘qeth_dbf_longtext’ might be a candidate for ‘gnu_printf’ format attribute [-Werror=suggest-attribute=format]
 6190 |         vsnprintf(dbf_txt_buf, sizeof(dbf_txt_buf), fmt, args);
      |         ^~~~~~~~~
Acked-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/s390/net/qeth_core.h

@@ -1084,6 +1084,7 @@ int qeth_setadpparms_set_access_ctrl(struct qeth_card *card,
 int qeth_do_ioctl(struct net_device *dev, struct ifreq *rq, int cmd);
 int qeth_siocdevprivate(struct net_device *dev, struct ifreq *rq,
 			void __user *data, int cmd);
+__printf(3, 4)
 void qeth_dbf_longtext(debug_info_t *id, int level, char *text, ...);
 int qeth_configure_cq(struct qeth_card *, enum qeth_cq);
 int qeth_hw_trap(struct qeth_card *, enum qeth_diags_trap_action);