ipvs: Avoid unnecessary calls to skb_is_gso_sctp

In the context of the SCTP SNAT/DNAT handler, these calls can only
return true.

Fixes: e10d3ba4 ("ipvs: Fix checksumming on GSO of SCTP packets")
Signed-off-by: Ismael Luceno <iluceno@suse.de>
Acked-by: Julian Anastasov <ja@ssi.bg>
Acked-by: Simon Horman <horms@kernel.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>