macsec: add rcu_barrier() on module exit

Without this, the various uses of call_rcu could cause a kernel panic.

Fixes: c09440f7

 ("macsec: introduce IEEE 802.1AE driver")
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>