IPVS: netns, defense work timer.

This patch makes defense work timer per name-space,
A net ptr had to be added to the ipvs struct,
since it's needed by defense_work_handler.

[ horms@verge.net.au: Use cancel_delayed_work_sync() instead of
	              cancel_rearming_delayed_work(). Found during
		      merge conflict resoliution ]
Signed-off-by: Hans Schillstrom <hans.schillstrom@ericsson.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>