pktgen: speedup fragmented skbs

We spend lot of time clearing pages in pktgen.
(Or not clearing them on ipv6 and leaking kernel memory)

Since we dont modify them, we can use one zeroed page, and get
references on it. This page can use NUMA affinity as well.

Define pktgen_finalize_skb() helper, used both in ipv4 and ipv6

Results using skbs with one frag :

Before patch :

Result: OK: 608980458(c608978520+d1938) nsec, 1000000000
(100byte,1frags)
  1642088pps 1313Mb/sec (1313670400bps) errors: 0

After patch :

Result: OK: 345285014(c345283891+d1123) nsec, 1000000000
(100byte,1frags)
  2896158pps 2316Mb/sec (2316926400bps) errors: 0

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>