ipvs: SIP fragment handling

Use the nfct_reasm SKB if available.

Based on part of a patch from: Hans Schillstrom
I have left Hans'es comment in the patch (marked /HS)

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
[ horms@verge.net.au: Fix comment style ]
Signed-off-by: Simon Horman <horms@verge.net.au>