sctp_diag: Respect ss adding TCPF_CLOSE to idiag_states

Since 'ss' always adds TCPF_CLOSE to idiag_states flags, sctp_diag can't
rely upon TCPF_LISTEN flag solely being present when listening sockets
are requested.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>