sctp: Fix the link time qualifier of 'sctp_ctrlsock_exit()'

The '.exit' functions from 'pernet_operations' structure should be marked
as __net_exit, not __net_init.

Fixes: 8e2d61e0

 ("sctp: fix race on protocol/netns initialization")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>