sfxge(4): avoid unnecessary mbuf data prefetch

Unnecessary prefetch just loads HW prefetcher and displaces other
cache entries (which could be really useful).

If we parse mbuf for TSO early and use firmware-assisted TSO, we do not
expect mbuf data access when we compose firmware-assisted TSO (v1 or v2)
option descriptors. If packet header needs to be linearized or finally
FATSO cannot be used because of, for example, too big header, we do not
care about a bit more performance degradation because of prefetch
absence (it is better to optimize more common case).

Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D9120