chtls: Avoid -Wflex-array-member-not-at-end warning

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

Use the `DEFINE_RAW_FLEX()` helper for on-stack definitions of
a flexible structure where the size of the flexible-array member
is known at compile-time, and refactor the rest of the code,
accordingly.

So, with these changes, fix the following warning:

drivers/net/ethernet/chelsio/inline_crypto/chtls/chtls_io.c:163:36: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Link: https://patch.msgid.link/aSQocKoJGkN0wzEj@kspp
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/ethernet/chelsio/inline_crypto/chtls/chtls_io.c

···
 int send_tx_flowc_wr(struct sock *sk, int compl,
 		     u32 snd_nxt, u32 rcv_nxt)
 {
-	struct flowc_packed {
-		struct fw_flowc_wr fc;
-		struct fw_flowc_mnemval mnemval[FW_FLOWC_MNEM_MAX];
-	} __packed sflowc;
+	DEFINE_RAW_FLEX(struct fw_flowc_wr, flowc, mnemval, FW_FLOWC_MNEM_MAX);
 	int nparams, paramidx, flowclen16, flowclen;
-	struct fw_flowc_wr *flowc;
 	struct chtls_sock *csk;
 	struct tcp_sock *tp;
 
 	csk = rcu_dereference_sk_user_data(sk);
 	tp = tcp_sk(sk);
-	memset(&sflowc, 0, sizeof(sflowc));
-	flowc = &sflowc.fc;
 
 #define FLOWC_PARAM(__m, __v) \
 	do { \