powerpc/mm: Replace zero-length array with flexible-array

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]

sizeof(flexible-array-member) triggers a warning because flexible array
members have incomplete type[1]. There are some instances of code in
which the sizeof operator is being incorrectly/erroneously applied to
zero-length arrays and the result is zero. Such instances may be hiding
some bugs. So, this work (flexible-array member conversions) will also
help to get completely rid of those sorts of issues.

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 76497732 ("cxgb3/l2t: Fix undefined behaviour")
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200507185755.GA15014@embeddedor

arch/powerpc/mm/hugetlbpage.c

@@ -253,7 +253,7 @@ int __init alloc_bootmem_huge_page(struct hstate *h)
 struct hugepd_freelist {
 	struct rcu_head	rcu;
 	unsigned int index;
-	void *ptes[0];
+	void *ptes[];
 };
 
 static DEFINE_PER_CPU(struct hugepd_freelist *, hugepd_freelist_cur);

tools/testing/selftests/powerpc/pmu/ebb/trace.h

@@ -18,7 +18,7 @@ struct trace_entry
 {
 	u8 type;
 	u8 length;
-	u8 data[0];
+	u8 data[];
 };
 
 struct trace_buffer
@@ -26,7 +26,7 @@ struct trace_buffer
 	u64  size;
 	bool overflow;
 	void *tail;
-	u8   data[0];
+	u8   data[];
 };
 
 struct trace_buffer *trace_buffer_allocate(u64 size);