SCTP: fix connection memory corruption

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>