[SCTP]: Fix the SO_REUSEADDR handling to be similar to TCP.

Update the SO_REUSEADDR handling to also check for listen state.  This
was muliple listening server sockets can't be created and they will
not steal packets from each other.

Reported by Paolo Galtieri <pgaltieri@mvista.com>

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>