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

Remove unused flexible-array member in struct tls_rec and, with this,
fix the following warning:

net/tls/tls.h:131:29: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Also, add a comment to prevent people from adding any members
after struct aead_request, which is a flexible structure --this is
a structure that ends in a flexible-array member.

Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Reviewed-by: Sabrina Dubroca <[e-mail]>
Link: https://patch.msgid.link/aNMG1lyXw4XEAVaE@kspp
Signed-off-by: Jakub Kicinski <[e-mail]>

net/tls/tls.h

···
 
 	char aad_space[TLS_AAD_SPACE_SIZE];
 	u8 iv_data[TLS_MAX_IV_SIZE];
+
+	/* Must be last --ends in a flexible-array member. */
 	struct aead_request aead_req;
-	u8 aead_req_ctx[];
 };
 
 int __net_init tls_proc_init(struct net *net);