macsec: add consistency check to netlink dumps

Use genl_dump_check_consistent in dump_secy.

Fixes: c09440f7 ("macsec: introduce IEEE 802.1AE driver")
Suggested-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>