tls: Fill software context without allocation

This patch allows tls_set_sw_offload to fill the context in case it was
already allocated previously.

We will use it in TLS_DEVICE to fill the RX software context.

Signed-off-by: Boris Pismenny <borisp@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>