macsec: ensure rx_sa is set when validation is disabled

macsec_decrypt() is not called when validation is disabled and so
macsec_skb_cb(skb)->rx_sa is not set; but it is used later in
macsec_post_decrypt(), ensure that it's always initialized.

Fixes: c09440f7

 ("macsec: introduce IEEE 802.1AE driver")
Signed-off-by: Beniamino Galvani <bgalvani@redhat.com>
Acked-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>