[SCTP]: Unhash the endpoint in sctp_endpoint_free().

This prevents a race between the close of a socket and receive of an
incoming packet.

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