encrypted-keys: 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 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

---
 include/keys/encrypted-type.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 9e9ccb20d586..38afb341c3f2 100644
@@ -27,7 +27,7 @@ struct encrypted_key_payload {
 	unsigned short payload_datalen;		/* payload data length */
 	unsigned short encrypted_key_format;	/* encrypted key format */
 	u8 *decrypted_data;	/* decrypted data */
-	u8 payload_data[0];	/* payload data + datablob + hmac */
+	u8 payload_data[];	/* payload data + datablob + hmac */
 };
 
 extern struct key_type key_type_encrypted;