e1000: convert to build_skb

Instead of preallocating Rx skbs, allocate them right before sending
inbound packet up the stack.

e1000-kvm, mtu1500, netperf TCP_STREAM:
Size   Size    Size     Time     Throughput
bytes  bytes   bytes    secs.    10^6bits/sec
old: 87380  16384  16384    60.00    4532.40
new: 87380  16384  16384    60.00    4599.05

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>