macsec: fix secy->n_rx_sc accounting

secy->n_rx_sc is supposed to be the number of _active_ rxsc's within a
secy. This is then used by macsec_send_sci to help decide if we should
add the SCI to the header or not.

This logic is currently broken when we create a new RXSC and turn it
off at creation, as create_rx_sc always sets ->active to true (and
immediately uses that to increment n_rx_sc), and only later
macsec_add_rxsc sets rx_sc->active.

Fixes: c09440f7 ("macsec: introduce IEEE 802.1AE driver")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: Antoine Tenart <atenart@kernel.org>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>