sctp: get netns from asoc and ep base

sctp: get netns from asoc and ep base

[ Upstream commit 4e7696d90b51a1a73ce0e8174f3aff58b914619c ]

Commit 312434617cb1 ("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>
Stable-dep-of: 1534ff77757e ("sctp: prevent possible shift-out-of-bounds in sctp_transport_update_rto")
Signed-off-by: Sasha Levin <sashal@kernel.org>