sctp: remove net sctp.x_enable working as a global switch

The netns sctp feature flags shouldn't work as a global switch,
which is mostly like a firewall/netfilter's job. Also, it will
break asoc as it discard or accept chunks incorrectly when net
sctp.x_enable is changed after the asoc is created.

Since each type of chunk's processing function will check the
corresp asoc's feature flag, this 'global switch' should be
removed, and net sctp.x_enable will only work as the default
feature flags for the future sctp sockets/endpoints.

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