NFS: 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: Anna Schumaker <Anna.Schumaker@Netapp.com>

include/linux/nfs4.h

@@ -38,7 +38,7 @@ struct nfs4_ace {
 
 struct nfs4_acl {
 	uint32_t	naces;
-	struct nfs4_ace	aces[0];
+	struct nfs4_ace	aces[];
 };
 
 #define NFS4_MAXLABELLEN	2048

include/linux/nfs_xdr.h

@@ -1227,7 +1227,7 @@ struct nfs4_secinfo4 {
 
 struct nfs4_secinfo_flavors {
 	unsigned int		num_flavors;
-	struct nfs4_secinfo4	flavors[0];
+	struct nfs4_secinfo4	flavors[];
 };
 
 struct nfs4_secinfo_arg {