tls: rx: strp: factor out copying skb data

We'll need to copy input skbs individually in the next patch.
Factor that code out (without assuming we're copying a full record).

Tested-by: Shai Amiram <samiram@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>