SCTP: API to add a sub-connection

This patch adds an API to add a sub-connection following a SRC/DST IP
mapping as required by the RFC4960.
At the same time, it changes the way the next available sub-connection
is being calculated: rather than having an index in the parent
connection which is prone to many issues at run-time, the next available
sub-connection is being calculated by looking at the state of the set
sub-connections and if marked as DOWN it means that is an available slot
to be used.

Change-Id: I662be6a247bfbbe8bf9aaf3f485183c07ef862fe
Signed-off-by: Marco Varlese <marco.varlese@suse.com>