tls: use fixed size for tls_offload_context_{tx,rx}.driver_state

driver_state is a flex array, but is always allocated by the tls core
to a fixed size (TLS_DRIVER_STATE_SIZE_{TX,RX}). Simplify the code by
making that size explicit so that sizeof(struct
tls_offload_context_{tx,rx}) works.

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>