SCTP: change sctp_ctl_sock_init() to try IPv4 if IPv6 fails

Change sctp_ctl_sock_init() to try IPv4 if IPv6 socket registration
fails.  Required if the IPv6 module is loaded with "disable=1", else
SCTP will fail to load.

Signed-off-by: Brian Haley <brian.haley@hp.com>
Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>