tls: rx: strp: set the skb->len of detached / CoW'ed skbs

alloc_skb_with_frags() fills in page frag sizes but does not
set skb->len and skb->data_len. Set those correctly otherwise
device offload will most likely generate an empty skb and
hit the BUG() at the end of __skb_nsg().

Fixes: 84c61fe1

 ("tls: rx: do not use the standard strparser")
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>