netdevsim: add dummy macsec offload

When the kernel is compiled with MACsec support, add the
NETIF_F_HW_MACSEC feature to netdevsim devices and implement
macsec_ops.

To allow easy testing of failure from the device, support is limited
to 3 SecY's per netdevsim device, and 1 RXSC per SecY.

v2:
 - nsim_macsec_add_secy, return -ENOSPC if secy_count isn't full but
   we can't find an empty slot (Simon Horman)
 - add sci_to_cpu to make sparse happy (Simon Horman)
 - remove set but not used secy variable (kernel test robot and
   Simon Horman)

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>