datapath: Always notify in initial namespace for port deletions.

datapath: Always notify in initial namespace for port deletions.

We currently notify for port deletions in the namespace of the device
that was deleted.  In general this should be initial namespace because
that's the only place where we look but it's possible that the device
was moved after being attached.  However, it's not semantically correct
because we really care about the namespace of the userspace process, not
that of the device.  This switches to genlmsg_multicast() which always
uses the initial namespace and seems more appropriate anyways.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>