sfxge(4): enforce packed stream fake buffer size

[arybchik]

Summary

In the case of packed stream real size of the buffer does not fit in
Rx descriptor byte count. Real size is specified on Rx queue setup.
Non-zero fake should be used to bypass hardware checks.

Sponsored by: Solarflare Communications, Inc.

Test Plan

amd64 tinderbox kernel builds tested
driver loaded/unloaded, interfaces up/down, traffic works fine