sflow: Set agent address properly based on collector address.

sflow: Set agent address properly based on collector address.

If an agent address is not provided, OVS tries to choose a source
address based on the source IP that would be used to connect to the
sFlow collector.  The code previously set the agent address to the
collector's address instead of using the calculated source address.
This patch properly uses the source address.

Reported-by: Neil McKee <neil.mckee@inmon.com>
Signed-off-by: Justin Pettit <jpettit@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>