[SCTP]: Verify all destination ports in sctp_connectx.

We need to make sure that all destination ports are the same, since
the association really must not connect to multiple different ports
at once.  This was reported on the sctp-impl list.

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>