pktgen: ipv6: numa: consolidate skb allocation to pktgen_alloc_skb

We currently allow for numa-node aware skb allocation only within the
fill_packet_ipv4() path, but not in fill_packet_ipv6(). Consolidate that
code to a common allocation helper to enable numa-node aware skb
allocation for ipv6, and use it in both paths. This also makes both
functions a bit more readable.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>