sctp: get netns from asoc and ep base

Commit 312434617c ("sctp: cache netns in sctp_ep_common") set netns
in asoc and ep base since they're created, and it will never change.
It's a better way to get netns from asoc and ep base, comparing to
calling sock_net().

This patch is to replace them.

v1->v2:
  - no change.

Suggested-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/output.c

@@ -282,7 +282,7 @@ static enum sctp_xmit sctp_packet_bundle_sack(struct sctp_packet *pkt,
 					sctp_chunk_free(sack);
 					goto out;
 				}
-				SCTP_INC_STATS(sock_net(asoc->base.sk),
+				SCTP_INC_STATS(asoc->base.net,
 					       SCTP_MIB_OUTCTRLCHUNKS);
 				asoc->stats.octrlchunks++;
 				asoc->peer.sack_needed = 0;