crypto: nx - Use static_assert() to check struct sizes

Commit 1e6b251c ("crypto: nx - Avoid -Wflex-array-member-not-at-end
warning") introduced tagged `struct nx842_crypto_header_hdr`. We want
to ensure that when new members need to be added to the flexible
structure, they are always included within this tagged struct.

So, we use `static_assert()` to ensure that the memory layout for
both the flexible structure and the tagged struct is the same after
any changes.
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

drivers/crypto/nx/nx-842.h

@@ -157,6 +157,7 @@ struct nx842_crypto_header_group {
 } __packed;
 
 struct nx842_crypto_header {
+	/* New members MUST be added within the struct_group() macro below. */
 	struct_group_tagged(nx842_crypto_header_hdr, hdr,
 		__be16 magic;		/* NX842_CRYPTO_MAGIC */
 		__be16 ignore;		/* decompressed end bytes to ignore */
@@ -164,6 +165,8 @@ struct nx842_crypto_header {
 	);
 	struct nx842_crypto_header_group group[];
 } __packed;
+static_assert(offsetof(struct nx842_crypto_header, group) == sizeof(struct nx842_crypto_header_hdr),
+	      "struct member likely outside of struct_group_tagged()");
 
 #define NX842_CRYPTO_GROUP_MAX	(0x20)