sctp: apply rhashtable api to sctp procfs

Traversal the transport rhashtable, get the association only once through
the condition assoc->peer.primary_path != transport.
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>