misc: 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")

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Dimitri Sivanich <sivanich@hpe.com>
Link: https://lore.kernel.org/r/20200226222240.GA14474@embeddedor
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/sgi-gru/grulib.h

@@ -136,7 +136,7 @@ struct gru_dump_context_header {
 	pid_t		pid;
 	unsigned long	vaddr;
 	int		cch_locked;
-	unsigned long	data[0];
+	unsigned long	data[];
 };
 
 /*

drivers/misc/sgi-gru/grutables.h

@@ -372,7 +372,7 @@ struct gru_thread_state {
 	int			ts_data_valid;	/* Indicates if ts_gdata has
 						   valid data */
 	struct gru_gseg_statistics ustats;	/* User statistics */
-	unsigned long		ts_gdata[0];	/* save area for GRU data (CB,
+	unsigned long		ts_gdata[];	/* save area for GRU data (CB,
 						   DS, CBE) */
 };