net/tls: don't look for decrypted frames on non-offloaded sockets

If the RX config of a TLS socket is SW, there is no point iterating
over the fragments and checking if frame is decrypted.  It will
always be fully encrypted.  Note that in fully encrypted case
the function doesn't actually touch any offload-related state,
so it's safe to call for TLS_SW, today.  Soon we will introduce
code which can only be called for offloaded contexts.

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>