Some cache related optimizations

1. prefetch 128 bytes of mbufs.
2. Re-order filling the pkt_info so cache stalls happen at the end
3. Define empty prefetch2cachelines() macro when the function isn't present.

Provides small performance improvments on some hardware

Reviewed by: sbruno
Approved by: sbruno (mentor)
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D12447