SCTP: Pick the correct port when binding to 0.

sctp_bindx() allows the use of unspecified port.  The problem is
that every address we bind to ends up selecting a new port if
the user specified port 0.  This patch allows re-use of the
already selected port when the port from bindx was 0.

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>