nfp: flower: handle acti_netdevs allocation failure

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SUSE CVE-2019-19081

A memory leak in the nfpflowerspawnvnicreprs function in drivers/net/ethernet/netronome/nfp/flower/main.c in the Linux kernel before 5.3.4 allows attackers to cause a denial of service memory consumption, aka CID-8ce39eb5a67a...