sctp: remove the typedef sctp_scope_policy_t

This patch is to remove the typedef sctp_scope_policy_t and keep
it's members as an anonymous enum.

It is also to define SCTP_SCOPE_POLICY_MAX to replace the num 3
in sysctl.c to make codes clear.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>