ipvs: create ip_vs_defrag_user

 	Create new function ip_vs_defrag_user to return correct
IP_DEFRAG_xxx user depending on the hooknum. It will be needed
when we add handlers in LOCAL_OUT.

Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>