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 <gustavoars@kernel.org>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://patch.msgid.link/aNMG1lyXw4XEAVaE@kspp


Signed-off-by: Jakub Kicinski <kuba@kernel.org>