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>

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 {