SCTP: fix connection memory corruption

A bug was found when multiple SCTP connections were being opened to the
same SCTP server. This patch addresses that problem, removing the use of
the 'parent' pointer approach for sub-connection and saving instead
within the sub-connection itself the ID representing its position. That
facilitates pointer-arithmetic to be computed in the
get_connection_from_transport().

Change-Id: Iaa1f4efc501590be1c93e42fd6fe3d6e02f635eb
Signed-off-by: Marco Varlese <marco.varlese@suse.com>

test/test_sctp.py

@@ -68,9 +68,10 @@ class TestSCTP(VppTestCase):
             self.logger.critical(error)
             self.assertEqual(error.find("failed"), -1)
 
-        error = self.vapi.cli("test echo client mbytes 10 appns 1" +
+        error = self.vapi.cli("test echo client nclients 2 mbytes 100" +
+                              " appns 1" +
                               " fifo-size 4" +
-                              " no-output test-bytes syn-timeout 20 " +
+                              " no-output test-bytes syn-timeout 3" +
                               " uri " + uri)
         if error:
             self.logger.critical(error)