net/tls: reorganize struct tls_context

struct tls_context is slightly badly laid out.  If we reorder things
right we can save 16 bytes (320 -> 304) but also make all fast path
data fit into two cache lines (one read only and one read/write,
down from four cache lines).

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>