sctp: fix /proc/net/sctp/ memory leak

Commit 13d782f6 ("sctp: Make the proc files per network namespace.")
changed the /proc/net/sctp/ struct file_operations opener functions to
use single_open_net() and seq_open_net().

Avoid leaking memory by using single_release_net() and seq_release_net()
as the release functions.

Discovered with Trinity (the syscall fuzzer).

Signed-off-by: Tommi Rantala <tt.rantala@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>