sctp: Set socket source address when additing first transport

Recent commits
	sctp: Get rid of an extra routing lookup when adding a transport
and
	sctp: Set source addresses on the association before adding transports

changed when routes are added to the sctp transports.  As such,
we didn't set the socket source address correctly when adding the first
transport.  The first transport is always the primary/active one, so
when adding it, set the socket source address.  This was causing
regression failures in SCTP tests.

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