sctp: remove rcu_read_lock in sctp_seq_dump_remote_addrs()

sctp_seq_dump_remote_addrs is only called by sctp_assocs_seq_show()
and it has been protected by rcu_read_lock that is from
rhashtable_walk_start().

So we will remove this one.

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