ipvs: Use IS_ERR_OR_NULL(svc) instead of IS_ERR(svc) || svc == NULL

This minor refactoring does not change the logic of function
ip_vs_genl_dump_dests.

Signed-off-by: Gao Feng <fgao@ikuai8.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>