sctp: support non-blocking version of the new sctp_connectx() API

Prior implementation of the new sctp_connectx() call that returns
an association ID did not work correctly on non-blocking socket.
This is because we could not return both a EINPROGRESS error and
an association id.  This is a new implementation that supports this.

Originally from Ivan Skytte Jørgensen <isj-sctp@i1.dk

Signed-off-by: Ivan Skytte Jørgensen <isj-sctp@i1.dk
Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>

net/sctp/associola.c

@@ -1470,6 +1470,10 @@ int sctp_assoc_set_id(struct sctp_association *asoc, gfp_t gfp)
 {
 	int assoc_id;
 	int error = 0;
+
+	/* If the id is already assigned, keep it. */
+	if (asoc->assoc_id)
+		return error;
 retry:
 	if (unlikely(!idr_pre_get(&sctp_assocs_id, gfp)))
 		return -ENOMEM;