s390: Replace zero-length array with flexible-array member

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 7649773293 ("cxgb3/l2t: Fix undefined behaviour")

Link: https://lkml.kernel.org/r/20200221150612.GA9717@embeddedor
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/appldata/appldata_os.c

@@ -75,7 +75,7 @@ struct appldata_os_data {
 				   (waiting for I/O)               */
 
 	/* per cpu data */
-	struct appldata_os_per_cpu os_cpu[0];
+	struct appldata_os_per_cpu os_cpu[];
 } __attribute__((packed));
 
 static struct appldata_os_data *appldata_os_data;

drivers/s390/block/dasd_diag.c

@@ -58,7 +58,7 @@ struct dasd_diag_private {
 
 struct dasd_diag_req {
 	unsigned int block_count;
-	struct dasd_diag_bio bio[0];
+	struct dasd_diag_bio bio[];
 };
 
 static const u8 DASD_DIAG_CMS1[] = { 0xc3, 0xd4, 0xe2, 0xf1 };/* EBCDIC CMS1 */

drivers/s390/block/dasd_eckd.h

@@ -220,7 +220,7 @@ struct LRE_eckd_data {
 	__u8 imbedded_count;
 	__u8 extended_operation;
 	__u16 extended_parameter_length;
-	__u8 extended_parameter[0];
+	__u8 extended_parameter[];
 } __attribute__ ((packed));
 
 /* Prefix data for format 0x00 and 0x01 */

drivers/s390/char/raw3270.h

@@ -211,7 +211,7 @@ struct string
 	struct list_head update;
 	unsigned long size;
 	unsigned long len;
-	char string[0];
+	char string[];
 } __attribute__ ((aligned(8)));
 
 static inline struct string *

drivers/s390/char/sclp_pci.c

@@ -39,7 +39,7 @@ struct err_notify_evbuf {
 	u8 atype;
 	u32 fh;
 	u32 fid;
-	u8 data[0];
+	u8 data[];
 } __packed;
 
 struct err_notify_sccb {

drivers/s390/cio/idset.c

@@ -13,7 +13,7 @@
 struct idset {
 	int num_ssid;
 	int num_id;
-	unsigned long bitmap[0];
+	unsigned long bitmap[];
 };
 
 static inline unsigned long bitmap_size(int num_ssid, int num_id)

drivers/s390/crypto/pkey_api.c

@@ -80,7 +80,7 @@ struct clearaeskeytoken {
 	u8  res1[3];
 	u32 keytype;	 /* key type, one of the PKEY_KEYTYPE values */
 	u32 len;	 /* bytes actually stored in clearkey[] */
-	u8  clearkey[0]; /* clear key value */
+	u8  clearkey[]; /* clear key value */
 } __packed;
 
 /*

drivers/s390/crypto/zcrypt_ccamisc.h

@@ -90,7 +90,7 @@ struct cipherkeytoken {
 	u16 kmf1;     /* key management field 1 */
 	u16 kmf2;     /* key management field 2 */
 	u16 kmf3;     /* key management field 3 */
-	u8  vdata[0]; /* variable part data follows */
+	u8  vdata[]; /* variable part data follows */
 } __packed;
 
 /* Some defines for the CCA AES cipherkeytoken kmf1 field */

drivers/s390/crypto/zcrypt_msgtype6.c

@@ -590,7 +590,7 @@ struct type86x_reply {
 	struct CPRBX cprbx;
 	unsigned char pad[4];	/* 4 byte function code/rules block ? */
 	unsigned short length;
-	char text[0];
+	char text[];
 } __packed;
 
 struct type86_ep11_reply {