e1000: switch to napi_gro_frags api

napi_gro_frags allows skb re-use in case GRO can merge payload pages
into an skb on the GRO lists.

netperf TCP_STREAM, kvm-e1000 emulation, mtu 9k:
Size   Size    Size     Time     Throughput
bytes  bytes   bytes    secs.    10^6bits/sec
old: 87380  16384  16384    30.00  8985.78
new: 87380  16384  16384    30.00  9907.05

Signed-off-by: Florian Westphal <fw@strlen.de>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>